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Advisory: Incorrect Handling of XSS Protection in ASP.Net
From: WebCohort Research (researchwebcohort.com)
Date: Mon Sep 08 2003 - 09:45:44 CDT
Monday, September 8th, 2003
As part of Microsoft's attempts to make it easier for application
developers to write secure code, Microsoft has added a new feature,
named Request Validation, to the ASP.Net 1.1 framework. This feature is
provides out of the box protection against Cross Site Scripting and
Script Injection attacks, by automatically checking all parameters in
the request and ensuring that their content does not include HTML tags.
WebCohort conducted research of this new ASP.Net feature, in order to
determine whether it actually provides protection against Cross Site
Scripting and Script Injection attacks or not.
The ASP.Net request validation feature has an implementation flaw, which
allows an attacker to easily bypass the content restrictions, possibly
exposing the application to Cross Site Scripting and Script Injection
Our research shows that the feature consists of banning all strings of
the form <letter from the content of parameters. Hence the string
"<script>", "<img" and even "<a>"are forbidden while strings like
"</script>" are allowed. When the server encounters a forbidden string
in the content of a parameter it issues an error message to the client.
As a result, WebCohort's Research Team was able to find a simple way to
bypass the filtering mechanism. This is done by placing a NULL character
between the less-then mark and the first character of the HTML Tag's
name. Since this is no longer recognized by the request validation
feature as a valid opening tag, it is ignored. However, many browsers,
including Microsoft's IE 6.0 disregard NULL characters in their input.
Hence when the string in interpreted by the browser it is interpreted as
an HTML tag, effectively yielding a Cross-Site Scripting (or Script
The exploit is done by simply adding a URL Encoded null character to the
request sent to the server. For instance:
Do not rely on this feature for Cross-Site Scripting or Script Injection
protection. The only effective method to avoid such attacks is
performing HTML encoding within the application code itself.
Microsoft was approached on Thursday, August 21st, and acknowledged the
problem the same day. According to Microsoft Security, an all-purpose
(non security) software update, due to be released in a few weeks, will
solve this problem. Since no preview of this update is currently
available, the update has not been tested by WebCohort Research.
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