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Posted Aug 24, 2005
Authored by Inge Henriksen

It is possible to remotely spoof the Microsoft Internet Information Server 5.0, 5.1 and 6.0 SERVER_NAME variable by doing a modified HTTP request. This allows for the revealing of sensitive ASP code through the IIS 500-100.asp error page.

tags | advisory, web, spoof, asp
SHA-256 | 9265062b769c12c9797d72a61a3d47995803db86c2d1079cb92eaf33f0bc6113


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** Inge Henriksen Security Advisory - Full Disclosure Proof of Concept at http://ingehenriksen.blogspot.com/ **

It is possible to remotely spoof the "SERVER_NAME" Microsoft® Internet Information Server® 5.0, 5.1 and 6.0 variable by doing a modified HTTP request. Thus potentially revealing sensitive ASP code through the IIS 500-100.asp error page, the spoof also opens up a potential range of exploits in third party web applications and web services.

Technical Description
Microsoft® Internet Information Server® shows sensitive information if the "SERVER_NAME" IIS 5.x server variable is "localhost". If a ASP page has an error, the code on the fault line in the ASP page is shown in the browser, but only if"SERVER_NAME" IIS server variable is "localhost". One can spoof the "SERVER_NAME" IIS server variable so that it shows whatever one want. Other third party web applications or web services may also be vulnerable if authentication depends on the validity this server variable.

The IIS server variable that can be spoofed is the "SERVER_NAME", it can be accessed through request.servervariables("SERVER_NAME") with ASP, and HttpContext.Current.Request.ServerVariables("SERVER_NAME") with .NET, other programming languages have other methods to access this server variable, but are equally vulnerable.

If the HTTP request comes from a remote client, then the server variable "SERVER_NAME" returns the IP address of the web server itself. If the HTTP request came from the same IP as the web server (thus the request came from the an authenticated user browsing from the web server itself), then request.servervariables("SERVER_NAME") returns "localhost". This fact is used as "proof" in web applications or web services that the person browsing the web server is in fact browsing from the web server itself. The web applications or web services may use this proof to display an administrative interface to the web browser user if such is the case. One exploitable example is the IIS 5.x 500-100.asp page; the page uses the server variable to determine if to display the code where the error occurred.

The technical description of the server variable is "The server's host name, DNS alias, or IP address as it would appear in self-referencing URLs", it is therefore often used to determine the IP address of the web server itself in once code, this opens up for a range of exploits including cookie-stealing, data redirection, and other URL manipulation issues.

There are many other applications that utilize validity of this server variable, and MSDN holds many examples for developers that are easily exploitable with this bug; among the most serious are the Microsoft® .NET Passport SDK examples, any web application or web service based on these examples are probably vulnerable due to the bug.

Vendor Status: Notified 28. January 2005

Confirmed applications: Microsoft® Internet Information Server® 5.0, Microsoft® Internet Information Server® 5.1 and Microsoft® Internet Information Server® 6.0.

Confirmed platforms: Microsoft® Windows® 2000 with SP4, Microsoft® Windows® XP Professional with SP2, Microsoft® Windows® 2003 with SP1.

Full Disclosure Proof of Consept at http://ingehenriksen.blogspot.com/
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