CTF: Shattering Prudentialv2

07/03/17 — capitol









During the Boston Key Party CTF 2017 we also solved the Prudentialv2 task.

We were presented with a simple website that asked us to log in, a peek into the html source showed us a link to what looked like the source code of the backend:

require 'flag.php';

if (isset($_GET['name']) and isset($_GET['password'])) {
    $name = (string)$_GET['name'];
    $password = (string)$_GET['password'];

    if ($name == $password) {
        print 'Your password can not be your name.';
    } else if (sha1($name) === sha1($password)) {
      die('Flag: '.$flag);
    } else {
        print '<p class="alert">Invalid password.</p>';

This might have looked like an impossible challenge, if it wasn’t for the fact that google had released an attack on sha1 a few days prior.

We first tried to url-encode the two pdf’s and upload them as username and password, but the server had a post limit of 8k.

But the effective attack on sha1 is really quite small, so we could use that instead.

One script to generate the username and password files:

#!/usr/bin/env python
# from https://github.com/lxe/shattered/blob/master/shattered.js

import hashlib
a = bytearray([
  0x73, 0x46, 0xdc, 0x91, 0x66, 0xb6, 0x7e, 0x11, 0x8f, 0x02, 0x9a, 0xb6, 0x21, 0xb2, 0x56, 0x0f,
  0xf9, 0xca, 0x67, 0xcc, 0xa8, 0xc7, 0xf8, 0x5b, 0xa8, 0x4c, 0x79, 0x03, 0x0c, 0x2b, 0x3d, 0xe2,
  0x18, 0xf8, 0x6d, 0xb3, 0xa9, 0x09, 0x01, 0xd5, 0xdf, 0x45, 0xc1, 0x4f, 0x26, 0xfe, 0xdf, 0xb3,
  0xdc, 0x38, 0xe9, 0x6a, 0xc2, 0x2f, 0xe7, 0xbd, 0x72, 0x8f, 0x0e, 0x45, 0xbc, 0xe0, 0x46, 0xd2,
  0x3c, 0x57, 0x0f, 0xeb, 0x14, 0x13, 0x98, 0xbb, 0x55, 0x2e, 0xf5, 0xa0, 0xa8, 0x2b, 0xe3, 0x31,
  0xfe, 0xa4, 0x80, 0x37, 0xb8, 0xb5, 0xd7, 0x1f, 0x0e, 0x33, 0x2e, 0xdf, 0x93, 0xac, 0x35, 0x00,
  0xeb, 0x4d, 0xdc, 0x0d, 0xec, 0xc1, 0xa8, 0x64, 0x79, 0x0c, 0x78, 0x2c, 0x76, 0x21, 0x56, 0x60,
  0xdd, 0x30, 0x97, 0x91, 0xd0, 0x6b, 0xd0, 0xaf, 0x3f, 0x98, 0xcd, 0xa4, 0xbc, 0x46, 0x29, 0xb1

b = bytearray([
  0x7f, 0x46, 0xdc, 0x93, 0xa6, 0xb6, 0x7e, 0x01, 0x3b, 0x02, 0x9a, 0xaa, 0x1d, 0xb2, 0x56, 0x0b,
  0x45, 0xca, 0x67, 0xd6, 0x88, 0xc7, 0xf8, 0x4b, 0x8c, 0x4c, 0x79, 0x1f, 0xe0, 0x2b, 0x3d, 0xf6,
  0x14, 0xf8, 0x6d, 0xb1, 0x69, 0x09, 0x01, 0xc5, 0x6b, 0x45, 0xc1, 0x53, 0x0a, 0xfe, 0xdf, 0xb7,
  0x60, 0x38, 0xe9, 0x72, 0x72, 0x2f, 0xe7, 0xad, 0x72, 0x8f, 0x0e, 0x49, 0x04, 0xe0, 0x46, 0xc2,
  0x30, 0x57, 0x0f, 0xe9, 0xd4, 0x13, 0x98, 0xab, 0xe1, 0x2e, 0xf5, 0xbc, 0x94, 0x2b, 0xe3, 0x35,
  0x42, 0xa4, 0x80, 0x2d, 0x98, 0xb5, 0xd7, 0x0f, 0x2a, 0x33, 0x2e, 0xc3, 0x7f, 0xac, 0x35, 0x14,
  0xe7, 0x4d, 0xdc, 0x0f, 0x2c, 0xc1, 0xa8, 0x74, 0xcd, 0x0c, 0x78, 0x30, 0x5a, 0x21, 0x56, 0x64,
  0x61, 0x30, 0x97, 0x89, 0x60, 0x6b, 0xd0, 0xbf, 0x3f, 0x98, 0xcd, 0xa8, 0x04, 0x46, 0x29, 0xa1

prefix= bytearray([
  0x25, 0x50, 0x44, 0x46, 0x2d, 0x31, 0x2e, 0x33, 0x0a, 0x25, 0xe2, 0xe3, 0xcf, 0xd3, 0x0a, 0x0a,
  0x0a, 0x31, 0x20, 0x30, 0x20, 0x6f, 0x62, 0x6a, 0x0a, 0x3c, 0x3c, 0x2f, 0x57, 0x69, 0x64, 0x74,
  0x68, 0x20, 0x32, 0x20, 0x30, 0x20, 0x52, 0x2f, 0x48, 0x65, 0x69, 0x67, 0x68, 0x74, 0x20, 0x33,
  0x20, 0x30, 0x20, 0x52, 0x2f, 0x54, 0x79, 0x70, 0x65, 0x20, 0x34, 0x20, 0x30, 0x20, 0x52, 0x2f,
  0x53, 0x75, 0x62, 0x74, 0x79, 0x70, 0x65, 0x20, 0x35, 0x20, 0x30, 0x20, 0x52, 0x2f, 0x46, 0x69,
  0x6c, 0x74, 0x65, 0x72, 0x20, 0x36, 0x20, 0x30, 0x20, 0x52, 0x2f, 0x43, 0x6f, 0x6c, 0x6f, 0x72,
  0x53, 0x70, 0x61, 0x63, 0x65, 0x20, 0x37, 0x20, 0x30, 0x20, 0x52, 0x2f, 0x4c, 0x65, 0x6e, 0x67,
  0x74, 0x68, 0x20, 0x38, 0x20, 0x30, 0x20, 0x52, 0x2f, 0x42, 0x69, 0x74, 0x73, 0x50, 0x65, 0x72,
  0x43, 0x6f, 0x6d, 0x70, 0x6f, 0x6e, 0x65, 0x6e, 0x74, 0x20, 0x38, 0x3e, 0x3e, 0x0a, 0x73, 0x74,
  0x72, 0x65, 0x61, 0x6d, 0x0a, 0xff, 0xd8, 0xff, 0xfe, 0x00, 0x24, 0x53, 0x48, 0x41, 0x2d, 0x31,
  0x20, 0x69, 0x73, 0x20, 0x64, 0x65, 0x61, 0x64, 0x21, 0x21, 0x21, 0x21, 0x21, 0x85, 0x2f, 0xec,
  0x09, 0x23, 0x39, 0x75, 0x9c, 0x39, 0xb1, 0xa1, 0xc6, 0x3c, 0x4c, 0x97, 0xe1, 0xff, 0xfe, 0x01

s1 = prefix + a
s2 = prefix + b

with open("user.blob", "w") as fp:

with open("pw.blob", "w") as fp:

And one script to upload them to the server and get the flag:

#!/usr/bin/env python

import socket
import urllib

user = open("user.blob").read()
password = open("pw.blob").read()

user = urllib.quote_plus(user)
password = urllib.quote_plus(password)

req = "GET /index.php?name=%s&password=%s HTTP/1.1\r\nHost:\r\n\r\n" % (user, password)

conn = socket.create_connection(("", 80))
print conn.recv(2048)

Flag: FLAG{AfterThursdayWeHadToReduceThePointValue}

Image credit