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I've read about Web Application Firewalls in a MOOC, and the provided example is that the WAF can filter out a request like ?user=<script to avoid potential XSS attacks.

But what if a webpage of the application allows to see a user's profile in a pretty way like view_user?name=... (instead of ID-based)? I could set my name to <script in the register form or so, and then, no one will be able to see my profile, because the page would legitimately be view_user?name=<script and the WAF rejects it?

Such username sounds odd, but it can be an advantage to have a non-accessible profile in some applications: like, in a game where you have to view the player's page [or a town's page or a character's page etc] to attack them, in a forum where you need to get to a user's page to edit/ban them, etc.

In more general terms: can you avoid side-effects of WAF rules that break business-logic? How?

  • It doesn't work like that in real world. Name=<script would never work because you view the profile of the person using unique identifier and not the name which is not unique. Regarding avoiding side-effects of WAF, this is done on the design stage of the application. By defining the business flow you write WAF rules specifically for it (every application uses dedicated WAF rules). And then you do testing of these rules to make sure it doesn't break anything. Also, WAF would not allow you to create name as <script> in first place. – Aria Oct 10 '17 at 12:14
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    @Aria I disagree. IMO, OP raises an actual issue with WAFs. They can occasionally be abused to trigger false positives. Also, a WAF might allow a username like <script> in a POST request but reject it in a URL. – Arminius Oct 10 '17 at 15:50
  • @Aria Hence my in a pretty way like view_user?name=... instead of ID-based. You can pass the ID still, and rely on it, using only the name as a decoration. Like /view/profile/<script-1234 (lot of websites do so, showing a page's title in the URL along with the real page ID). And so, as @Arminius said, I was actually wondering if WAF can be abused to trigger false positives, what consequences this can lead to and how to deal with these/avoid these. – Xenos Oct 11 '17 at 14:57
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In a general sense, WAFs are historically notorious for false positives- issuing alerts on legitimate requests when in non-blocking mode, and for edge case breakage of applications, difficult or impossible to reproduce in non-production environments, when in blocking mode.

As a result, the overall space is evolving away from signature- and granular request-specific alerting- which is really the responsibility of the application- and moving more towards reputation analysis, statistical anomaly detection, and providing pooled intelligence around the behavior of threat actors.

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