Biscuits

Momma, can I have cookie..?

No....

Author: d4y0n3

21KB

main

Arch:       amd64-64-little
RELRO:      Full RELRO
Stack:      Canary found
NX:         NX enabled
PIE:        PIE enabled
SHSTK:      Enabled
IBT:        Enabled
Stripped:   No

A simple challenge where you have to predict the RNG picking an extremely large array of Cookie names, I got a local solve but the remote has disgustingly bad latency that it never got the correct RNG predict.

  local_10 = *(long *)(in_FS_OFFSET + 0x28);
  tVar2 = time((time_t *)0x0);
  srand((uint)tVar2);
  puts("Give me the cookie I want a 100 times in a row and I\'ll gi ve you the flag!");
  fflush(stdout);
  for (local_f8 = 0; local_f8 < 100; local_f8 = local_f8 + 1) {
    iVar1 = rand();
    strcpy(local_78,*(char **)(cookies + (long)(iVar1 % 100) * 8) );
    printf("Guess the cookie: ");
    fflush(stdout);
    fgets(local_e8,100,stdin);
    sVar3 = strcspn(local_e8,"\n");
    local_e8[sVar3] = '\0';
    iVar1 = strcmp(local_e8,local_78);
    if (iVar1 != 0) {
      printf("Wrong. The cookie I wanted was: %s\n",local_78);
                    /* WARNING: Subroutine does not return */
      exit(0);
    }
    printf("Correct! The cookie was: %s\n",local_78);
    fflush(stdout);
  }

Decompiled using Ghidra, this program wants the user to predict the correct cookie name 100 times in a row to get the flag. First step is to find a list of cookie names using Ghidra, which I did by looking at the cookie array pointer and copied everything to ChatGPT to translate it to a Python array:

Here is the RNG prediction and cookie input loop script:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# -*- template: winterbitia -*-

# ====================
# -- PWNTOOLS SETUP --
# ====================

from pwn import *
from time import time
from ctypes import CDLL

exe = context.binary = ELF(args.EXE or 'main')
trm = context.terminal = ['tmux', 'splitw', '-h']

host = args.HOST or 'chals.bitskrieg.in'
port = int(args.PORT or 6000)

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):
    '''Connect to the process on the remote host'''
    io = connect(host, port)
    if args.GDB:
        gdb.attach(io, gdbscript=gdbscript)
    return io

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)

gdbscript = '''
tbreak main
continue
'''.format(**locals())

# =======================
# -- EXPLOIT GOES HERE --
# =======================

io = start()
log.info(io.recvline())
libc = CDLL('/lib/x86_64-linux-gnu/libc.so.6')
libc.srand(libc.time(0))

cookies = [
    "Chocolate Chip", "Sugar Cookie", "Oatmeal Raisin", "Peanut Butter", "Snickerdoodle", "Shortbread", "Gingerbread", 
    "Macaron", "Macaroon", "Biscotti", "Butter Cookie", "White Chocolate Macadamia Nut", "Double Chocolate Chip", 
    "M&M Cookie", "Lemon Drop Cookie", "Coconut Cookie", "Almond Cookie", "Thumbprint Cookie", "Fortune Cookie", 
    "Black and White Cookie", "Molasses Cookie", "Pumpkin Cookie", "Maple Cookie", "Espresso Cookie", "Red Velvet Cookie", 
    "Funfetti Cookie", "S'mores Cookie", "Rocky Road Cookie", "Caramel Apple Cookie", "Banana Bread Cookie", 
    "Zucchini Cookie", "Matcha Green Tea Cookie", "Chai Spice Cookie", "Lavender Shortbread", "Earl Grey Tea Cookie", 
    "Pistachio Cookie", "Hazelnut Cookie", "Pecan Sandies", "Linzer Cookie", "Spritz Cookie", "Russian Tea Cake", 
    "Anzac Biscuit", "Florentine Cookie", "Stroopwafel", "Alfajores", "Polvorón", "Springerle", "Pfeffernüsse", 
    "Speculoos", "Kolaczki", "Rugelach", "Hamantaschen", "Mandelbrot", "Koulourakia", "Melomakarona", "Kourabiedes", 
    "Pizzelle", "Amaretti", "Cantucci", "Savoiardi (Ladyfingers)", "Madeleine", "Palmier", "Tuile", "Langue de Chat", 
    "Viennese Whirls", "Empire Biscuit", "Jammie Dodger", "Digestive Biscuit", "Hobnob", "Garibaldi Biscuit", 
    "Bourbon Biscuit", "Custard Cream", "Ginger Nut", "Nice Biscuit", "Shortcake", "Jam Thumbprint", "Coconut Macaroon", 
    "Chocolate Crinkle", "Pepparkakor", "Sandbakelse", "Krumkake", "Rosette Cookie", "Pinwheel Cookie", 
    "Checkerboard Cookie", "Rainbow Cookie", "Mexican Wedding Cookie", "Snowball Cookie", "Cranberry Orange Cookie", 
    "Pumpkin Spice Cookie", "Cinnamon Roll Cookie", "Chocolate Hazelnut Cookie", "Salted Caramel Cookie", 
    "Toffee Crunch Cookie", "Brownie Cookie", "Cheesecake Cookie", "Key Lime Cookie", "Blueberry Lemon Cookie", 
    "Raspberry Almond Cookie", "Strawberry Shortcake Cookie", "Neapolitan Cookie"
]

for i in range(100):
    rng = libc.rand() % 100
    cookie = cookies[int(rng)]
    io.sendlineafter("cookie: ", bytes(cookie, 'utf-8'))
    log.info(f"Sent cookie: {cookie}")
    log.info(io.recvline())

io.interactive()