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Details:

   PHP 4.2.0 introduced a completely rewritten multipart/form-data POST 
   handler. While I was working on the code in my role as PHP developer
   i found a bug within the way the mime headers are processed. 
   A malformed POST request can trigger an error condition, that is not
   correctly handled. Due to this bug it could happen that an uninit-
   ialised struct gets appended to the linked list of mime headers.
   When the lists gets cleaned or destroyed PHP tries to free the pointers
   that are expected in the struct. Because of the lack of initialisation
   those pointers contain stuff that was left on the stack by previous
   function calls. 

   On the IA32 architecture (aka. x86) it is not possible to control what
   will end up in the uninitialised struct because of the stack layout. All 
   possible code paths leave illegal addresses within the struct and PHP
   will crash when it tries to free them.

   Unfortunately the situation is absolutely different if you look on a
   solaris sparc installation. Here it is possible for an attacker to free
   chunks of memory that are full under his control. This is most probably
   the case for several more non IA32 architectures.

   Please note that exploitability is not only limited to systems that are
   running malloc()/free() implementations that are known to be vulnerable
   to control structure overwrites. This is because the internal PHP memory
   managment implements its own linked list system that can be used to
   overwrite nearly arbitrary memory addresses.
   

Proof of Concept:

   e-matters is not going to release the exploit for this vulnerability to
   the public.
   

Vendor Response:

   22th July 2002 - An updated version of PHP which fixes this 
                    vulnerability was released and can be downloaded at:

                       http://www.php.net/downloads.php

                    The vendor announcement is available at:

                       http://www.php.net/release_4_2_2.php


Recommendation:

   If you are running PHP 4.2.x you should upgrade as soon as possible,
   especially if your server runs on a non IA32 CPU. If you cannot upgrade
   for whatever reason the only way to workaround this, is to disable all
   kinds of POST requests on your server.  
   
   
GPG-Key:

   http://security.e-matters.de/gpg_key.asc
    
   pub  1024D/75E7AAD6 2002-02-26 e-matters GmbH - Securityteam
   Key fingerprint = 43DD 843C FAB9 832A E5AB  CAEB 81F2 8110 75E7 AAD6


Copyright 2002 Stefan Esser. All rights reserved.