October 04, 2013

Score sheet: Testing Some XSS Evasion Techniques Against Our WAF

IStock_000016474569SmallA couple of months ago ModSecurity (SpiderLabs) issued an “XSS Evasion Challenge” where they actively asked security experts and hackers to try and bypass their own XSS filters. This is a blessed initiative by a security company, to go and find loopholes in their engines in order to improve on their security posture.

The exercise included two main challenges:

  • Evading the ModSecurity WAF regex (CRS)
  • Evading client side protection “sandbox”

Per the challenge results, ModSecurity filters were updated to add the logic that is required to block these evasion techniques.

With security in mind, we were interested in checking the evasion techniques against our SecureSphere WAF its default configuration. Here are the results:

Evasion Technique #1: - “Nul Bytes” – Blocked out of the box.


Evasion Technique #2: Sandbox Evasion (MentalJS) – Blocked out of the box.


Bottom line – We were able to block both evasion techniques out of the box. Our customers are protected.

We also agree with SpiderLabs’s conclusions:

  • Blacklisting attacks with signatures is not enough. We agree. Part of the reason that IPS/NGFW and manually configured WAF solutions don’t solve web application attacks is that they rely on blacklists.  Effective protection against application attacks requires a positive security model to understand how the application works. See our Blog on “What the IPS Didn’t See”.
  • SpiderLabs say that Regular Expression matching (Signatures) is not enough. We agree. Imperva’s Technology does not rely on RegExp matching alone, but also incorporates smart engines that are able to understand the attack’s logic.



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