PHP open_basedir Race Condition Vulnerability

Advisory Content :

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Hardened-PHP Project
www.hardened-php.net

-= Security Advisory =-

Advisory: PHP open_basedir Race Condition Vulnerability
Release Date: 2006/10/04
Last Modified: 2006/10/04
Author: Stefan Esser [sesser (at) hardened-php (dot) net [email
concealed]]

Application: PHP 4/5
Not affected: PHP with Suhosin Extension 0.9.6
Severity: A design flaw of open_basedir allows bypassing it
with the symlink() function
Risk: Critical
References: http://www.hardened-php.net/advisory_082006.132.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."

The design of the open_basedir feature of PHP that is meant to
disallow access to files outside a set of configured directories
is vulnerable to race conditions.

It was discovered that this design flaw can be exploited with
the usage of PHP's symlink() function in a very easy way.
We believe that the only solution to this problem is disabling
the function symlink() while open_basedir is used (this feature
was therefore added to our Suhosin PHP Security Extension).

Fixing the flaw in the open_basedir design seems infeasible
because there is no way to fix the potential race condition
when accessing the file is done within an external library.

The successful exploitation of this vulnerability allows access
to files normally not accessible due to the open_basedir
restriction.

Details:

PHP's open_basedir feature is meant to disallow scripts to access
files outside a set of configured base directories. The checks
for this are placed within PHP functions dealing with files before
the actual open call is performed.

Obviously there is a little span of time between the check and the
actual open call. During this time span the checked path could
have been altered and point to a file that is forbidden to be
accessed due to open_basedir restrictions.

Because the open_basedir restrictions often not call PHP functions
but 3rd party library functions to actually open the file it is
impossible to close this time span in a general way. It would only
be possible to close it when PHP handles the actual opening on it's
own.

While it seems hard to change the path during this little time span
it is very simple with the use of the symlink() function combined
with a little trick. PHP's symlink() function ensures that source
and target of the symlink operation are allowed by open_basedir
restrictions (and safe_mode). However it is possible to point a
symlink to any file by the use of mkdir(), unlink() and at least
two symlinks.

Example (pseudo PHP code):

mkdir("a/a/a/a/a/a");
symlink("a/a/a/a/a/a", "dummy");
symlink("dummy/../../../../../../", "xxx");
unlink("dummy");
symlink(".", "dummy");

After this code sequence "xxx" points to 6 directories up. Having
achieved this it is possible for an attacker to exploit the race
condition by creating 2 PHP scripts.

The first script alternates a symbolic link between a file that
is allowed and the one that is forbidden by open_basedir and the
second script simply puts loops around operations trying to
operate on the symbolic link.

The result is that sometimes in the loop the race is lost and the
operation is performed on the allowed file, sometimes it just
produces errors, because the symlink was deleted and sometimes it
triggers the open_basedir error-message. However sooner or later
the race will be won and the operation is performed on the file
that is actually forbidden due to open_basedir restrictions.

Proof of Concept:

The Hardened-PHP Project is not going to release exploits for
this vulnerability to the public.

Disclosure Timeline:

02. October 2006 - Notified security (at) php (dot) net [email concealed]
04. October 2006 - Public Disclosure

Recommendation:

Because the design flaw cannot be solved it is strongly recommended
to disable the symlink() function if you are using the open_basedir
feature. You can achieve that by adding symlink to the list of
disabled functions within your php.ini

disable_functions=...,symlink

Additionally you should start thinking about not relying on PHP's
open_basedir and safe_mode restrictions but on actual operating
system features like chroots and jails, because open_basedir and
safe_mode are simply insecure by design.

As usual we also strongly recommend to install the latest version
of our Suhosin Extension. The current version 0.9.6 disallows
symlink() while open_basedir is used by default. Additionally it
comes with configurable function black- and white-lists that
can work (unlike disable_functions) on a per virtual host basis.

Grab your copy and more information at:

http://www.hardened-php.net/suhosin/index.html

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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PHP open_basedir Race Condition Vulnerability