[Full-disclosure] Advisory 13/2006: PHP HTML Entity Encoder Heap Overflow Vulnerability

2006-11-03T00:00:00
ID SECURITYVULNS:DOC:14908
Type securityvulns
Reporter Securityvulns
Modified 2006-11-03T00:00:00

Description

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1

                    Hardened-PHP Project
                    www.hardened-php.net

                  -= Security  Advisory =-


 Advisory: PHP HTML Entity Encoder Heap Overflow Vulnerability

Release Date: 2006/11/03 Last Modified: 2006/11/03 Author: Stefan Esser [sesser@hardened-php.net]

Application: PHP 5 <= 5.1.6, PHP 4 <= 4.4.4 Severity: Bufferoverflows in htmlentities() and htmlspecialchars() may result in arbitrary remote code execution Risk: Critical Vendor Status: Vendor has released PHP 5.2.0 which fixes this issue References: http://www.hardened-php.net/advisory_132006.138.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."

While we were searching for a hole in htmlspecialchars() and htmlentities() to bypass the encoding of certain chars to exploit a possible eval() injection hole in another application we discovered that the implementation contains a possible bufferoverflow that can be triggered when the UTF-8 charset is selected.

Unfortunately the whole purpose of both functions is to prepare userinput for HTML output. Therefore they are used in most PHP applications as protection against XSS and are always exposed to userinput.

By triggering the overflow it is possible to overwrite heap management structures with a limited charset. This can result in remote code execution. Exploitability has been proven against for example Linux with glibc 2.3 in a test environment. It depends on the heap layout, the OS heap implementation and the used Zend Memory Manager.

Details:

The HTML entity encoder of PHP will increase the size of it's output buffer every time it reaches the end of the current buffer. Unfortunately the check assumes that the maximum length of an HTML entity is 8 chars, which is true for most entities. However especially the Greek character set contains entities that are longer than 8 chars. Because of this it is for example possible to trigger the overflow by embedding Greek theta UTF-8 characters into the input string.

Because the longest HTML entity currently supported is 10 bytes long this allows overflowing the buffer with the 2 bytes ';' and '\0'. When exploiting heap overflows it can be enough to just overwrite the appending memory structure with a single '\0' char and control the content of the following memory block to execute arbitrary code.

While the above Greek character exploit is only possible in the htmlentities() function it is also possible to overwrite with up to 7 chars by embedding broken UTF-8 characters into the string. The characters may come from the limited charset 0x00, 0xc0-0xfd.

On Linux glibc systems this is for example enough to trick realloc into believing that the next memory block is empty and long enough to store the additional 128 bytes. The position of the buffer is therefore not changed and following writes to the output buffer will overwrite the Zend Memory Manager structure of the following block. This allows the typical linked list unlink exploit against the Zend Memory Manager.

Proof of Concept:

The Hardened-PHP Project is not going to release a proof of concept exploit for this vulnerability.

Disclosure Timeline:

  1. October 2006 - Notified security@php.net, patch in CVS
  2. November 2006 - Notified vendor-sec
  3. November 2006 - PHP developers released PHP 5.2.0
  4. November 2006 - Public Disclosure

Recommendation:

For PHP 4 users it is strongly recommended to patch their version of PHP with the following patch until php.net is providing PHP4 updates.

http://cvs.php.net/viewvc.cgi/php-src/ext/standard/html.c?r1=1.63.2.23.2.2&r2=1.63.2.23.2.3&view=patch

As usual we very strongly recommend that you install Suhosin-Patch and the Suhosin Extension, because once again this advisory proved that remotely triggerable overflows in PHP still exist. It is therefore highly recommended by us to use Suhosin-Patch. It's canary protection will detect overflows and stop execution to make exploitation very hard or impossible.

FreeBSD and OpenBSD's PHP ports already come with Suhosin-Patch activated by default.

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-2006-5465 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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