Advisory 19/2005: PHP register_globals Activation Vulnerability in parse_str()

[Stefan Esser <sesser@xxxxxxxxxxxxxxxx>]

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                        Hardened-PHP Project
                        www.hardened-php.net

                      -= Security  Advisory =-


     Advisory: PHP register_globals Activation Vulnerability in parse_str()
 Release Date: 2005/10/31
Last Modified: 2005/10/31
       Author: Stefan Esser [sesser@xxxxxxxxxxxxxxxx]

  Application: PHP4 <= 4.4.0
               PHP5 <= 5.0.5
     Severity: Unsafe termination of parse_str() may result in the 
               register_globals directive turned back on
         Risk: Low
Vendor Status: Vendor has released a bugfixed PHP 4 version
   References: http://www.hardened-php.net/advisory_192005.78.html


Overview:

   PHP is a widely-used general-purpose scripting language that is
   especially suited for Web development and can be embedded into HTML.

   During the development of the Hardening-Patch which adds security 
   hardening features to the PHP codebase, several vulnerabilities 
   within PHP were discovered. This advisory describes one of these 
   flaws concerning a weakness in the implementation of the parse_str()
   function. Under certain conditions triggering the memory_limit
   request shutdown during a parse_str() call will result in the core
   of PHP believing that the register_globals directive is turned on
   (for the rest of the lifetime of the involved webserver process).

   This may allow an attacker to exploit security flaws in PHP 
   applications that exist due to uninitialised global variables.


Details:

   When parse_str() is called with only one parameter it parses the
   supplied string, as if it were the query string passed via a URL 
   and sets variables in the global scope. This is achieved by 
   internally switching register_globals on, while the string is
   parsed.
   
   Unfortunately it could be possible for an external attacker to 
   trigger the memory_limit request termination during such a call
   to parse_str() by sending a lot of request variables to consume
   enough memory to trigger the limit. (It is described elsewhere
   how it is possible to consume a lot of memory with a small 
   request body). If the request shutdown is executed during the
   call to parse_str() the register_globals flag is left on, for
   the rest of the lifetime of the involved webserver process.
   
   Because the flag is only internally changed and this has nothing
   todo with setting ini variables, the script is not able to detect
   that register_globals is on in an easy way. This tricks a lot of
   register_globals deregistration layers, because they usually
   only get activated when the ini_get() functions returns that
   register_globals is turned on.
   
   This vulnerability is rated low, because calls to parse_str() 
   with only one parameter are very rare. Additionally even if
   register_globals is turned on without the script knowing, this
   is only a security problem if the affected script does not
   properly intialise it's variables.


Proof of Concept:

   The Hardened-PHP project is not going to release exploits for any
   of these vulnerabilities to the public.


Recommendation:

   It is strongly recommended to upgrade to the new PHP-Releases as
   soon as possible, because it also fixes a few vulnerabilities,
   that are rated critical. Additionally we always recommend to
   run PHP with the Hardening-Patch applied.


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 2005 Stefan Esser. All rights reserved.

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