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Serious PHP memory_limit remote vulnerability

Apache2 users who use php and limites how much memory a script may consume with memory_limit in php.ini should consider upgrading to php version 4.3.7, previous versions may allow remote attackers to execute arbitrary code.

The hole affects all apache2 installations using PHP <= 4.3.7 and PHP5 <= 5.0.0RC3.

The hole was, like most Open Source vulnerabilities, plugged extremely quickly and packages for most distributions were available before it was publicly released.

  • Gentoo Users: PHP 4.3.7 has been available in portage since 2004-07-04. emerge mod_php php to upgrade.
  • Suse users: Updated packages for Suse became available 2004-07-16, SUSE-SA_2004_021.txt.
  • The Full e-matters GmbH Security Advisory: 112004.txt
                             e-matters GmbH
                        -= Security  Advisory =-
       Advisory: PHP memory_limit remote vulnerability
   Release Date: 2004/07/14
  Last Modified: 2004/07/14
         Author: Stefan Esser []
    Application: PHP <= 4.3.7
                 PHP5 <= 5.0.0RC3
       Severity: A vulnerability within PHP allows remote code
                 execution on PHP servers with activated memory_limit
           Risk: Critical
  Vendor Status: Vendor has released a bugfixed version.
     PHP is a widely-used general-purpose scripting language that is 
     especially suited for Web development and can be embedded into HTML.
     According to Security Space PHP is the most popular Apache module
     and is installed on about 50% of all Apaches worldwide. This figure
     includes of course only those servers that are not configured with
     During a reaudit of the memory_limit problematic it was discovered 
     that it is possible for a remote attacker to trigger the memory_limit
     request termination in places where an interruption is unsafe. This
     can be abused to execute arbitrary code on remote PHP servers.
     On the 28th June 2004 Gregori Guninski released his advisory about
     a possible remote DOS vulnerability within Apache 2 (CAN-2004-0493).
     This vulnerability allows tricking Apache 2 into acception arbitrary
     sized HTTP headers. Guninski and many others rated this bug as "Low
     Risk" for 32bit systems, but they did not take into account that 
     such a bug could have a huge impact on 3rd party modules.
     After his advisory was released I reaudited PHP's memory_limit 
     request termination, because this bug made it possible to reach the 
     memory_limit at places that were never meant to be interrupted. 
     After a possible exploitation path for Apache 2 servers was 
     discovered and a working exploit was created, similar pathes were 
     found and added to the proof of concept exploit that allowed
     exploitation of NON Apache 2 servers. (f.e. Apache 1.3.31)
     The idea of the exploit is simple. When PHP allocates a block of
     memory it first checks in the cache of free memory blocks for a block
     of the same size. If such a block is found it is taken from the cache
     otherwise PHP checks if an allocation would violate the memory_limit.
     In that case the request shutdown is triggered through zend_error(). 
     (PHP < 4.3.7 aborts after the violating memory block is allocated)
     PHP contains several places where such an interruption is unsafe.
     An example for such places are those where Zend HashTables are 
     allocated and initialised. This is performed in 2 steps and the
     initialisation step itself allocates memory before important members
     are correctly initialised. An attacker that is able to trigger the
     memory_limit abort within zend_hash_init() and is additionally able
     to control the heap before the HashTable itself is allocated, is 
     able to supply his own HashTable destructor pointer.
     Several places within PHP where found where this action is performed
     on HashTables that actually get destructed by the request shutdown.
     One of such places is f.e. within the fileupload code, but is only 
     triggerable on Apache 2 servers that are vulnerable to CAN-2004-0493, 
     another one is only reachable if variables_order was changed to have 
     the "E" in the end, a third one is within session extension which is 
     activated by default but the vulnerability can not be triggered if
     the session functionality is not used. A fourth place is within the
     implementation of the register_globals functionality. Although this
     is deactivated by default since PHP 4.2 it is activated on nearly
     all servers that have to ensure compatibility with older scripts.
     Other places might exist in not default activated or 3rd party
     All mentioned places outside of the extensions are quite easy to
     exploit, because the memory allocation up to those places is 
     deterministic and quite static throughout different PHP versions.
     The only unknown entity is the size of the environment vars array.
     But that is usually small and can be bruteforced with some kind
     of binary search algorithm. Additionally this information could
     leak to an attacker through an open phpinfo() page. If the admin
     used php.ini-recommended as configuration basis it is irrelevant
     anyway because the ENV array is not populated in that case.
     Because the exploit itself consist of supplying an arbitrary 
     destructor pointer this bug is exploitable on any platform.
     (Except the system runs with non exec heap+stack protection)
     This includes systems running Hardened-PHP <= 0.1.2 because they
     have no protection of the HashTable destructor pointer.
     As a last word it should be said, that an attacker does not need
     to send 8/16/64MB (or whatever the memory_limit is) per attack.
     With POST requests it is quite easy to eat 100 (and more) times 
     the amount of sent bytes.
  Proof of Concept:
     e-matters is not going to release an exploit for this vulnerability
     to the public.
  Disclosure Timeline:
     07. July 2004 - Vendor-sec was informed about the fact that this
                     vulnerability was found
     14. July 2004 - Public Disclosure
  CVE Information:
     The Common Vulnerabilities and Exposures project ( has
     assigned the name CAN-2004-0594 to this issue.
     If you are running PHP with compiled in memory_limit support, it is
     strongly recommended that you upgrade as soon as possible to the 
     newest version. Disabling memory_limit within your configuration can
     be considered a workaround, but leaves your site vulnerable to 
     memory hungry PHP scripts or POST requests that create huge variables.
     If you are running PHP with Apache <= 2.0.49 ensure that you have the
     fix for CAN-2004-0493 applied.
     pub  1024D/3004C4BC 2004-05-17 e-matters GmbH - Securityteam 
     Key fingerprint = 3FFB 7C86 7BE8 6981 D1DA  A71A 6F7D 572D 3004 C4BC
  Copyright 2004 Stefan Esser. All rights reserved.

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