0x0 Introduction

Feeling lucky?

You must create a flag.txt in the same folder as the binary for it to run.
nc ctf.b01lers.com 9202

Author: robotearthpizza
Difficulty: Easy

0x1 Mitigation

Arch:     amd64-64-little
RELRO:    Full RELRO
Stack:    Canary found
NX:       NX enabled
PIE:      PIE enabled

0x2 Vulnerability

function sym.casino ask for a 4 byte string using fgets, then it compare with a random string generate by sym.imp.rand().

If we enter the same string as the random one, we get certain amount of money. If we have more than 1000 in balance, the program will print out the flag

{
    do {
        // generate random string
        sym.imp.fgets(&s2, 5, _reloc.stdin);
        iVar1 = sym.imp.strcmp(&s1);
        // add or subtract balance depend on the result
    } while (_obj.balance < 1000);
    sym.give_flag();
    return;
}

The vulnerability here is that the binary never set random seed using srand, therefore, the sequence rand() give us will always be the same.

we can use following code to generate string and send string to the server

#include <stdlib.h>
#include <stdio.h>

int main () {
    char s[5];
    s[4] = '\x0';
    for (int j=0;j<200;j++) {
        for (int i=0;i<4;i++) {
            int x = rand();
            s[i] = x + (x / 0x1a) * -(0x1a) + 'a';
        }
        printf("\"%s\",",s);
    }
}

0x3 Exploit

from pwn import *

values = ["nwlr","bbmq","bhcd","arzo","wkky","hidd","qscd","xrjm","owfr","xsjy","bldb","efsa","rcby","necd","yggx","xpkl","orel","lnmp","apqf","wkho","pkmc","oqhn","wnku","ewhs","qmgb","buqc","ljji","vswm","dkqt","bxix","mvtr","rblj","ptns","nfwz","qfjm","afad","rrws","ofsb","cnuv","qhff","bsaq","xwpq","cace","hchz","vfrk","mlno","zjkp","qpxr","jxki","tzyx","acbh","hkic","qcoe","ndto","mfgd","wdwf","cgpx","iqvk","uytd","lcgd","ewht","acio","hord","tqkv","wcsg","spqo","qmsb","oagu","wnny","qxnz","lgdg","wpbt","rwbl","nsad","eugu","umoq","cdru","beto","kyxh","oach","wdvm","xxrd","ryxl","mndq","tukw","agml","ejuu","kwci","bxub","umen","meya","tdrm","ydia","jxlo","ghiq","fmzh","lvih","jouv","suyo","ypay","ulye","imuo","tehz","riic","fskp","ggkb","bipz","zrzu","cxam","ludf","ykgr","uowz","gioo","obpp","leql","wpha","pjna","dqhd","cnvw","dtxj","bmyp","ppha","uxns","pusg","dhii","xqmb","fjxj","cvud","jsuy","ibye","bmws","iqyo","ygyx","ymze","vypz","vjeg","ebeo","cfuf","tsxd","ixti","gsie","ehkc","hzdf","lilr","jqfn","xztq","rsvb","spky","hsen","bppk","qtpd","dbuo","tbbq","cwiv","rfxj","ujjd","dntg","eiqv","dgai","jvwc","yaub","wewp","jvyg","ehlj","xepb"]

io = connect("ctf.b01lers.com", 9202)
i = 0
try:
    while True:
        lp(io.sendlineafter(b"letters: ",values[i].encode()))
        i+=1
except:
    pass
print(io.recv())

io.interactive()

0x4 Flag

forgot