tags: TSG CTF 2021

TSG CTF 2021 Beginner's Rev Author's Writeup

author: @m1kit

Problem Overview

You are given a binary that tells you if a flag is correct or not.

Hints

• Hint 1: Try to run the given program. You should get somehow helpful information about the flag.
• Hint 2: Do you know ghidra? To understand the behavior of the program, you need to do some static analysis.
• Hint 3: Don't spend too much on reading the code. Once you get an idea of the behavior, I recommend you to try some dynamic analysis with various tools.
• Hint 4:

  There are some interesting system calls.
• Hint 5:

  32 characters, 32 processes
• Hint 6:

  You can use strace to observe system calls.

🙈 Solution below

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Solution

Run

As mentioned in the hint, we can run the program first.

❯ ./beginners_rev
give me exactly one argument

❯ ./beginners_rev aaa
give me an argument with 32 chars

❯ ./beginners_rev aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
wrong

Now we know this programs determine correctness of something with 32 chars length.

Read

Next, let's read the program with ghidra.

We can see the entrypoint like this.
It calls check only if it has 32 chars.

undefined8 main(int param_1,long param_2) {
  char *__s;
  size_t sVar1;
  undefined8 uVar2;
  
  if (param_1 == 2) {
    __s = *(char **)(param_2 + 8);
    sVar1 = strlen(__s);
    if (sVar1 == 0x20) {
      uVar2 = check(__s);
      return uVar2;
    }
    puts("give me an argument with 32 chars");
  }
  else {
    puts("give me exactly one argument");
  }
  return 1;
}

Let's see check.
There are some interesting system calls!

byte check(long param_1) {
  __pid_t _Var1;
  int iVar2;
  int iVar3;
  int iVar4;
  uint uVar5;
  long in_FS_OFFSET;
  byte bVar6;
  undefined local_34;
  byte local_33;
  long local_30;
  
  uVar5 = 0;
  iVar4 = 0;
  iVar3 = 0;
  local_30 = *(long *)(in_FS_OFFSET + 0x28);
  do {
    _Var1 = fork();
    iVar4 = iVar4 + 1;
    if (_Var1 == 0) {
      iVar4 = 0;
      uVar5 = uVar5 | 1 << ((byte)iVar3 & 0x1f);
      iVar2 = open("/dev/null",1);
      dup2(iVar2,1);
    }
    iVar3 = iVar3 + 1;
  } while (iVar3 != 5);
  iVar3 = iVar4 + -1;
  iVar2 = is_correct((int)*(char *)(param_1 + (int)uVar5),uVar5);
  bVar6 = iVar2 == 0;
  if (iVar4 != 0) {
    do {
      iVar3 = iVar3 + -1;
      wait(&local_34);
      bVar6 = bVar6 | local_33;
    } while (iVar3 != -1);
  }
  if (bVar6 == 0) {
    puts("correct");
  }
  else {
    puts("wrong");
  }
  if (local_30 == *(long *)(in_FS_OFFSET + 0x28)) {
    return bVar6;
  }
                    /* WARNING: Subroutine does not return */
  __stack_chk_fail();
}

The key point is the program calls fork() five times in for-loop.
Therefore we have 32 processes, and each of them checks one character in the flag around line number 29.

Hope this rough illust of 4 processes example will help you.

After that results from processes are aggregates around line number 35.

Each process prints the result to stdout at line number 38-43, however, only the message from the root process is visible since anything else is redirected to /dev/null at line number 23.

Solve

With strace command, you can observe system calls executed.
We can use this to check the number of times puts("correct"); has been called.

strace -f ./dist/beginners_rev "$1" 2>&1 | grep correct | wc -l

We can create a simple brute force script to determine the flag.
Note that you need to check from the back to the front, due to the structure of the process tree.

import subprocess
import string

def oracle(v):
  p = subprocess.Popen(["./the_magic_oracle_above.sh", v], stdout=subprocess.PIPE)
  output, _ = p.communicate()
  return int(str(output, 'ascii'))

ans = ['*'] * 32
for i in range(31,-1,-1):
  for c in string.printable:
    ans[i] = c
    flag = ''.join(ans)
    if oracle(flag) == 32 - i:
      break
print(flag)

Alt 1

@moratorium08's solution

Alternatively, you can patch the binary to remove dup2(open("/dev/null", 1),1);.
Now you can see the number of "correct"s in your stdout.

Alt 2

@taiyoslime's solution

You can write a gdb script to leak a hint for the flag.
But you need some efforts to avoid anti-debugging codes in is_correct function.

FYI: this is my anti-debugging mechanism.

if (__builtin_return_address(0) - (void *)check != 0x5f) {
    fputs("This function may not work properly with a debugger.", stderr);
}

Appendix

Here's our original C code for check.

int check(char *flag)
{
  int index = 0, count = 0;
  for (int i = 0; i < 5; i++) {
    if (fork()) {
      count++;
    } else {
      count = 0;
      index |= 1 << i;
      dup2(open("/dev/null", O_WRONLY), 1);
    }
  }

  int result = !is_correct(flag[index], index);
  while (count--) {
    int status;
    wait(&status);
    result |= WEXITSTATUS(status);
  }

  if (result == 0) {
    printf("correct\n");
  } else {
    printf("wrong\n");
  }
  return result;
}