<<< Date Index >>>     <<< Thread Index >>>

Remote IIS 5.x and IIS 6.0 Server Name Spoof



** 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/