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



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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@hardened-php.net]

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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