Airlock WAF 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


found: 2012-04-05

by: G. Wagner

SEC Consult Vulnerability Lab




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.





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:









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:









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.






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



Advisory URL:





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EOF G. Wagner / @2012