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Detect Hash Colisions Attach on ASP .net
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UPDATE: Microsoft has a Fix

Microsoft has published a fix, using a diferent approch as used here. They are limiting the number of post parameters to exactly 1000.

To install this update, use Windows Update or visit the link belowe:

This project however can be used for study of the IIS Modules and for future problems.


The Problem

This is a simple IIS 7 module to avoid the zero day vulnerability which targets string hash colisions discovered by Alexander "alech" Klink and Julian "zeri" Wälde [1] which is described in a security advisory [2].

There was a security advisory [2] of Microsoft describing the problem and a blog post explaing in details the problem [3].

The Proposed Solution

This solution consists in expection the request input before ASP .net and detecting duplicated form post hashes.

If the form post is "big" ann contains a duplicated hash then the request is rejected.

I used some reference to implement this module. [5]


This solution needs more work, I only released it now to share the discovery with the community and to receive comments.

I will add more checks and check more data at input, it will be released until the end of the week.

Please send any comments to

The vulnerability is better described in these links:

[1] [2] [3] [4] [5]