Airlock WAF 4.2.4 - Overlong UTF-8 Sequence Bypass

SEC Consult Vulnerability Lab Security Advisory < 20120618-1 >
=======================================================================
title: Airlock WAF overlong UTF-8 sequence bypass
product: Airlock
vulnerable version: <= 4.2.4 (without hotfix HF4213)
fixed version: 4.2.5
impact: critical
homepage: http://www.ergon.ch/
found: 2012-04-05
by: G. Wagner
SEC Consult Vulnerability Lab
https://www.sec-consult.com/
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Vendor description:
-------------------

Airlock is a Web application firewall (WAF) that offers a combination of
defence mechanisms for Web applications. It has been developed to meet the
security standards criteria of the payment card industry (PCI DSS), online
banking security and the protection of e-commerce, and provides sustainable,
easy to administrate and audit security for Web applications.

source: http://www.ergon.ch/en/airlock/

Vulnerability overview/description:
-----------------------------------

The Airlock WAF protection can be completely bypassed by using overlong UTF-8
character representations of the NUL character such as C0 80, E0 80 80 and F0
80 80 80. During the tests no internal knowledge of the WAF was known, but it
is suspected that the UTF-8 decoder fails to reject the overlong NUL byte
character representations and they get decoded as U+0000 later on. Further the
WAF would not perform any checks for attack patterns after the NUL byte.

Proof of concept:
-----------------

The WAF blocks requests based on patterns that it recognizes as a web
application attack, for instance:

allowed: 
http://www.abc.com/file.ext?id=101'+union+select

forbidden:
http://www.abc.com/file.ext?id=101'+union+select+1+from+Dual+--+

If an overlong UTF-8 character representation of the NUL byte forgoes any web
application attack sequence, the WAF would fail to recognize the attack
pattern, for instance:

forbidden:
http://www.abc.com/file.ext?id=101'+union+select+col1,col2,col3+from+tab
le+--+

allowed:
http://www.abc.com/file.ext?id=101%C0%80'+union+select+col1,col2,col3+fr
om+table+--+

Vendor contact timeline:
------------------------
2012-04-05: Informed customer about vulnerability in their Airlock WAF.
2012-06-08: Received permission from customer to contact vendor directly. 
2012-06-11: Contacted security contact at Ergon with vulnerability information.
2012-06-13: Vendor indicates that issue is already fixed. They had received
vulnerability information already by the customer.
2012-06-18: Vendor provides information on patched versions.

Solution:
---------

The vendor resolved the issue on the 19th of April 2012 with Update 4.2.5 or
the Hotfix HF4213 for the versions 4.2.4 and 4.2.3.3.

Advisory URL:
-------------

https://www.sec-consult.com/en/advisories.html

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EOF G. Wagner / @2012