Microsoft is investigating a reported vulnerability in its ASP.Net that could allow an attacker to bypass security features on a Web server and view sensitive content.

The software giant has published an advisory on the vulnerability with instructions for lessening the effects of the hole, which is in a component of ASP.Net called the "canonicalisation routine". Modified URLs can be used to exploit the flaw, and so bypass authentication on Web applications that use ASP.Net, Microsoft said.

ASP.Net is the latest version of Microsoft's Active Server Page (ASP) technology, which allows developers to create Web-based applications by embedding small programs, written in a variety of languages such as Visual Basic, Perl and C#, in Web pages, which are then transmitted to users.

The canonicalisation routine is a process that interprets URL requests for content on a Web server and determines the proper way to respond for those requests. For example, there is more than one way to refer to the location of a file, such as homepage.html, on a Web server. The canonicalisation routine interprets the information in a URL to determine what page the URL is asking for, according to information on the Microsoft Web page.

The vulnerability affects all versions of ASP.Net, including versions running on Windows 2000, Windows 2000 Server, XP Professional and Windows Server 2003.

Microsoft has released a small software module called the Microsoft ASP.Net ValidatePath module (VPModule.msi) that can be applied to Internet Information Server 5.0, 5.1 or 6.0 Web servers to prevent exploitation of the ASP.Net vulnerability. It is working on a security patch.

The company also recommended that website owners review its knowledge base for suggestions on ways to program checks for problems with URL canonicalization.