We have received numerous public replies as well as several private emails.
Thanks for your comments, suggestions and feedbacks.
It's now time to summarize and clarify some points.
Q: Is this a new class of exploits or just another case of applications lacking input validation?
A: Actually, HPP is an input validation flaw. As SQL Injection and XSS, we may consider it as an injection weakness.
In this specific case, query string delimiters are the "dangerous" characters.
Q: You are saying that several HTTP back-ends manage multiple occurrences in different ways. In some cases, it may be abused in order to fingerprint the underline back-end. Is it right?
A: Yes, sure. However, considering the granularity available, we don't think it is really so interesting.
Q: This is a known attack. You guys presented a bunch of interesting but already known techniques to exploit different vulnerabilities.
A: Actually, we think we have contributed (in some way) to the current state-of-art showing this issue. However, even if it is currently used by 'hard-core' attackers, it's very important to formalize a threat in order to mitigate the issue and create efficient workarounds.
The aim of the entire research is to raise awareness around this problem.
In future, we would like to include HPP within the OWASP Testing Guide in order to provide the right methodology for testing systems against HPP-like attacks as well.
We strongly believe that sharing such knowledge may increase the security of all web applications.
Q: Most of your examples and findings use GET parameters. What about POST?
A: POST and COOKIE parameters may be affected as well. In slide #11 and #19, we have briefly stated that and you will see further research because it is a very interesting aspect since it gives additional flexibility for all attacks.
Q: In the current version of IE8, is the XSS Filter still vulnerable to HPP?
A: No! We had a discussion with the IE XSS Filter guy at Microsoft and turns out that the current version is NOT affected. All previous tests were done against the beta release and we didn't double check the latest one. We are sorry for this misunderstanding.
Q: Are multiple occurrences of a parameter valid according to the RFC, W3C, whatever?
A: Yes! Yes! The only thing which in fact was worth mentioning is the lack of standard in the _management_ of multiple occurrences and NOT the presence of multiple occurrences themselves.
After all, that's why it is possible to abuse the query string delimiters injection flaw.
Q: Is Yahoo! Mail still vulnerable to HPP?
A: Difficult to say. However, the specific issue was patched thus it cannot be abused by malicious users.
Q: Could you provide additional details regarding the Yahoo! Classic Mail HPP attack?
A: We've just published here an in-depth review of the issue with the video PoC as well.
Q: What's the right way of managing multiple occurrences? Is there a 'perfect' framework?
A: No, there are no right o wrong behaviors as well as we cannot refer to a right or wrong web servers/web frameworks. The behavior of the HTTP back-ends is a matter of exploitability, only.
Q: HPP is only about WAFs bypasses?
A: Absolutely not! HPP is also about applications flow manipulation, anti-CSRF, content pollution.
Q: How can I prevent HPP?
A: First of all, answer yourself "Which layer am I protecting?".
Then, speaking about HPP server side, it's always important to use URL encoding whenever you do GET/POST HTTP requests to an HTTP back-end.
From the client-side point of view, use URL encoding whenever you are going to include user-supplied content within links, etc.
Q: Am I vulnerable to HPP?
A: It depends on how you are managing several occurrences of the same parameter from the application point of view. Using strict input validation checkpoints and the right output filtering (URL encoding), you are likely secure (at least, against HPP :p).
That's all (for now).
