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2 HTTP Policy Filter Options

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Recently there was some talk about the following two HTTP filter options so I thought maybe more people would also be interested.

Verify normalization:

 

If you want to block requests with URLs containing escaped characters after normalization.

 

Web servers receive requests that are URL encoded. This means that certain characters may be replaced with a percent sign (%) followed by a particular number. For example, %20 corresponds to a space, so a request for http://myserver/My%20Dir/My%20File.htm is the same as a request for http://myserver/My Dir/My File.htm. Normalization is the process of decoding URL-encoded requests.

 

Because the % can be URL encoded, an attacker can submit a carefully crafted request to a server that is basically double-encoded. If this occurs, Internet Information Services (IIS) may accept a request that it would otherwise reject as not valid. When you select Verify Normalization, the HTTP filter normalizes the URL two times. If the URL after the first normalization is different from the URL after the second normalization, the filter rejects the request. This prevents attacks that rely on double-encoded requests.

Note that while we recommend that you use the Verify Normalization function, it may also block legitimate requests that contain a %.

 

 

Block high bit characters:

 

Select Block high bit characters to specify that URLs with high-bit characters will be blocked. This can help block some attacks on Web servers running Internet Information Services (IIS), but may also block requests and responses that contain characters from one of several languages that require high-bit characters.

 

 

For more information on HTTP filtering see the following TechNet article.

Gershon Levitz

ISA Server User Education

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