UrlScan v3.0 filtering based on Request Entity

While some folks are rejoicing, others are noticing the lack of scanning for the request entity. Why would we do that? The easy answer is that this is just not possible with an ISAPI filter. In IIS 5 and earlier, there is no API that would allow us to filter request entity, so request entity will never happen there.  In IIS 6, the APIs exist to do this via * script maps, but the performance would be pretty bad.  Also, there is no mechanism to treat the request as a stream, so there would be potential memory utilization problems. So we left it at that and said "can't do it, sorry".

But then you ask, "what about the request filtering module in IIS7"? Theoretically the IIS7 module APIs certainly let you analyze the request entity, so it is certainly possible. One of our current tasks is to bring the request filtering module up to par (feature-wise) with UrlScan v3.0 and then we can consider answering this complex problem. Yes, don't let this fool you ... analyzing request entity is a complex problem that has consequences for both performance and security. There is a multitude of things you need to account for here: compression/encryption, custom serialization, signature split between multiple POSTs, memory pressure due to entity buffering for POST data, etc. By no means are we claiming that this is an impossible task ... just that the cost to benefit ratio for this is low at this point. But we will try to look into this, time permitting.

Please feel free to send your thoughts/comments ... happy filtering !!

Published Monday, June 30, 2008 9:21 PM by naziml

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