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collision

{"author": ["ret2basic"]}

Challenge

Daddy told me about cool MD5 hash collision today. I wanna do something like that too!
ssh [email protected] -p2222 (pw:guest)

Code Review

#include <stdio.h>
#include <string.h>
unsigned long hashcode = 0x21DD09EC;
int* ip = (int*)p;
int i;
int res=0;
for(i=0; i<5; i++){
res += ip[i];
}
return res;
}
int main(int argc, char* argv[]){
if(argc<2){
printf("usage : %s [passcode]\n", argv[0]);
return 0;
}
if(strlen(argv[1]) != 20){
printf("passcode length should be 20 bytes\n");
return 0;
}
system("/bin/cat flag");
return 0;
}
else
printf("wrong passcode.\n");
return 0;
}
The program takes 5 4-byte hex numbers as `argv[1]` and it calls `check_password()`. If the sum of these 5 hex numbers equals `0x21DD09EC` then the program prints out the flag.

Solution

We need to find 5 integers in hex that sum to `0x21dd09ec`. The naive idea is to divide `0x21dd09ec` by 5. It won't work because `0x21dd09ec` isn't divisible by 5. But, following from this logic, we could find a solution using the formula:
0x21dd09ec = (0x21dd09ec // 5) * 4 + <residue>
Here we have `0x21dd09ec // 5 = 0x6c5cec8`, so `residue = 0x21dd09ec - 0x6c5cec8 * 4 = 0x6c5cecc`. Since the convertion is done by `int* ip = (int*)p` (pointer, not string), we should use little-endian in `argv[1]`.

Exploit

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# This exploit template was generated via:
# \$ pwn template --host pwnable.kr --port 2222 --user col --password guest --path /home/col/col
from pwn import *
# Set up pwntools for the correct architecture
exe = context.binary = ELF('col')
# Many built-in settings can be controlled on the command-line and show up
# in "args". For example, to dump all data sent/received, and disable ASLR
# for all created processes...
# ./exploit.py DEBUG NOASLR
# ./exploit.py GDB HOST=example.com PORT=4141
host = args.HOST or 'pwnable.kr'
port = int(args.PORT or 2222)
user = args.USER or 'col'
remote_path = '/home/col/col'
# Connect to the remote SSH server
shell = None
if not args.LOCAL:
shell = ssh(user, host, port, password)
def start_local(argv=[], *a, **kw):
'''Execute the target binary locally'''
if args.GDB:
return gdb.debug([exe.path] + argv, gdbscript=gdbscript, *a, **kw)
else:
return process([exe.path] + argv, *a, **kw)
def start_remote(argv=[], *a, **kw):
'''Execute the target binary on the remote host'''
if args.GDB:
return gdb.debug([remote_path] + argv, gdbscript=gdbscript, ssh=shell, *a, **kw)
else:
return shell.process([remote_path] + argv, *a, **kw)
def start(argv=[], *a, **kw):
'''Start the exploit against the target.'''
if args.LOCAL:
return start_local(argv, *a, **kw)
else:
return start_remote(argv, *a, **kw)
# Specify your GDB script here for debugging
# GDB will be launched if the exploit is run via e.g.
# ./exploit.py GDB
gdbscript = '''
tbreak main
continue
'''.format(**locals())
#===========================================================
# EXPLOIT GOES HERE
#===========================================================
# Arch: i386-32-little
# RELRO: Partial RELRO
# Stack: Canary found
# NX: NX enabled
# PIE: No PIE (0x8048000)
argv = [p32(0x6c5cec8) * 4 + p32(0x6c5cecc)]
print(f"{argv = }")
io = start(argv)