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Date: Thu, 5 May 2005 22:06:21 +0200
From: Michal Zalewski <>
Subject: Re: ASP.NET __VIEWSTATE crypto validation prone to replay attacks

> Microsoft has addressed your issues 1-a, 1-b and 1-c by adding a
> property "ViewStateUserKey" to the System.Web.UI.Page class in
> .NET Framework 1.1.  The documentation for this property is here:

No, I think this statement is simply not true. The key is intended to
solve a wholly different class of problems, and has nothing to do with
replay attacks I reported.

Reported issue 1-a: same user moves __VIEWSTATE from one page to
another. Adding unique user identifier to __VIEWSTATE does not help,
because in both cases, it's precisely the same user within the same

Reported issue 1-b: same user retains __VIEWSTATE and reuses it at a
later date. Again, it's precisely the same person and the same unique

Reported issue 1-c: this is the least relevant of all reported
problems, and indeed can be in some situations mitigated by binding
__VIEWSTATE data to a specific user.


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