Pwnable.kr-[collision] writeup

Daddy told me about cool MD5 hash collision today.
I wanna do something like that too!

ssh [e-mail] -p2222 (pw:guest)

col@pwnable:~$ ls -l
total 16
-r-sr-x--- 1 col_pwn col     7341 Jun 11  2014 col
-rw-r--r-- 1 root    root     555 Jun 12  2014 col.c
-r--r----- 1 col_pwn col_pwn   52 Jun 11  2014 flag
col@pwnable:~$ cat col.c
#include <stdio.h>
#include <string.h>
unsigned long hashcode = 0x21DD09EC;
unsigned long check_password(const char* p){
    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;
    }

    if(hashcode == check_password( argv[1] )){
        system("/bin/cat flag");
        return 0;
    }
    else
        printf("wrong passcode.\n");
    return 0;
}

コマンドラインからの２０バイトの引数を５つの４バイトの整数として分け、その和が0x21dd09ecとなるようにします。

from pwn import *
frag = (0x21dd09ec + 1) // 5
payload = frag.to_bytes(4,'little')*4
payload += (frag-1).to_bytes(4,'little')

s1 = ssh(host='pwnable.kr', user='col',password='guest', port=2222)
r1 = s1.process(executable='./col', argv=['fd', payload])
print(r1.recv())
s1.close()

$ python3 collision_pwn.py
[+] Connecting to pwnable.kr on port 2222: Done
[*] [e-mail]:
    Distro    Ubuntu 16.04
    OS:       linux
    Arch:     amd64
    Version:  4.4.179
    ASLR:     Enabled
[+] Starting remote process './col' on pwnable.kr: pid 66019
b'daddy! I just managed to create a hash collision :)\n'
[*] Closed connection to 'pwnable.kr'