PayloadsAllTheThings/PHP juggling type

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

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
MD5	240610708	0e462097431906509019562988736854	Michal Spacek
SHA1	10932435112	0e07766915004133176347055865026311692244	Independently found by Michael A. Cleverly & Michele Spagnuolo & Rogdham

<?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'));
?>

References

• Writing Exploits For Exotic Bug Classes: PHP Type Juggling By Tyler Borland
• Magic Hashes - WhieHatSec