Advisory 09/2006: PHP unserialize() Array Creation Integer Overflow



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Hardened-PHP Project
www.hardened-php.net

-= Security Advisory =-


Advisory: PHP unserialize() Array Creation Integer Overflow
Release Date: 2006/10/09
Last Modified: 2006/10/09
Author: Stefan Esser [sesser@hardened-php.net]

Application: PHP 5 <= 5.1.6, PHP 4 < 4.3.0
Not affected: PHP 4 >= 4.3.0,
PHP with Hardening-Patch,
PHP with Suhosin-Patch
Severity: User-input passed to the unserialize() function might
trigger an integer overflow in array creation that
might result in remote code execution
Risk: Critical
Vendor Status: Fixed in CVS, no security update planned, wait for PHP 5.2.0
References: http://www.hardened-php.net/advisory_092006.133.html


Overview:

Quote from http://www.php.net
"PHP is a widely-used general-purpose scripting language that
is especially suited for Web development and can be embedded
into HTML."

The PHP 5 branch of the PHP source code lacks the protection
against possible integer overflows inside ecalloc() that is
present in the PHP 4 branch and also for several years part of
our Hardening-Patch and our new Suhosin-Patch.

It was discovered that such an integer overflow can be triggered
when user input is passed to the unserialize() function. Earlier
vulnerabilities in PHP's unserialize() that were also discovered
by one of our audits in December 2004 are unrelated to the newly
discovered flaw, but they have shown, that the unserialize()
function is exposed to user-input in many popular PHP applications.
Examples for applications that use the content of COOKIE variables
with unserialize() are phpBB and Serendipity.

The successful exploitation of this integer overflow will result
in arbitrary code execution.


Details:

Several years ago when integer overflows inside the calloc()
function-family became popular in the security industry, many
function libraries that contained similar allocation functions
were hardened against the possible multiplication overflow.

The ecalloc() function used in the Zend Engine 1 was one of the
functions that were hardened against this kind of attack.
Unfortunately the PHP developers never bothered merging this
protection into the code of the Zend Engine 2, which powers PHP 5.

However when Hardened-PHP was created by us in 2004 a similar
protection of ecalloc() was added to our patch. Therefore our
users are safe from the problem described here.

unserialize() is a PHP function that allows deserialisation of
previously serialised PHP variables. Many applications use it
to store PHP variables in an easy accessible data format. Some
of them even put serialised strings into COOKIEs and later
unserialize() them again.

Unfortunately passing user-input to unserialize() is a bad idea
because it also supports the deserialisation of objects, which
might influence the application. Additionally it is trivial to
crash PHP with a large amount of nested arrays.

Furthermore we discovered that storing a large value for the
number of array elements into the serialised string will trigger
an integer overflow inside ecalloc(), resulting in the allocation
of 0 bytes (plus the Zend Memory Manager management headers).
When this happens unserialize() will continue working with a
to small bucket array. When the string is parsed for the array
elements the HashTable functions will operate on memory not
allocated for this purpose. This can lead to several different
memory corruption attacks.

A carefully crafted string can use this to execute arbitrary code.
In our proof of concept exploit this is achieved by storing a
HashTable with our own destructor into the PHP array. When the
PHP variable is destroyed this will result in the execution of
our shellcode.

Note: In general this kind of attack on HashTable destructors
is impossible when Hardening-Patch or Suhosin-Patch are installed
because both detect modification of HashTable destructors.

Note: In general we dislike to release advisories while there are
no official fixed versions available, but the fact that Linux
distributions already are shipping patched versions and that the
PHP CVS contains an explicit commit message we believe it is
important all parties about unserialize(). It is not really a
problem to find out with a few grep commands that unserialize()
is allowing dangerous ecalloc() calls.


Proof of Concept:

The Hardened-PHP Project will release a proof of concept exploit
for this vulnerability after the release of PHP 5.2.0 has happened
and a few weeks have passed.


Disclosure Timeline:

30. September 2006 - Notified security@php.net, vendor-sec
30. September 2006 - Patch was committed to PHP CVS
05. October 2006 - Redhat, Mandriva release PHP updates
06. October 2006 - PHP security updates in media
09. October 2006 - Public Disclosure


Recommendation:

It is strongly recommended to patch your version of PHP with the
following patch until php.net is providing updates.

http://www.hardened-php.net/files/CVE-2006-4812.patch

As usual we very strongly recommend to install our Suhosin-Patch
and the Suhosin Extension. Once again it was proved, that our
patch protects users of PHP against unknown flaws within PHP.
Users of our patch have been protected against this flaw for about
2 years now.

Our Suhosin extension on the other hand supports transparent
cookie encryption. When TCE is activated, which is the default
setting, and the encryption key is set it is not possible for
an external attacker to exploit the unserialize() flaws in our
examples: phpBB and Serendipity.

Grab your copy and more information at:

http://www.hardened-php.net/suhosin/index.html


CVE Information:

The Common Vulnerabilities and Exposures project (cve.mitre.org) has
assigned the name CVE-2006-4812 to this vulnerability.


GPG-Key:

http://www.hardened-php.net/hardened-php-signature-key.asc

pub 1024D/0A864AA1 2004-04-17 Hardened-PHP Signature Key
Key fingerprint = 066F A6D0 E57E 9936 9082 7E52 4439 14CC 0A86 4AA1


Copyright 2006 Stefan Esser. All rights reserved.

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