.NET Form Authentication Insecure Redirect

`Seeker Research Center Security Advisory  
By Irene Abezgauz   
  
===========  
I. Overview  
===========  
  
An Insecure Redirect vulnerability has been identified in the .NET Form  
Authentication - in the Redirect From Login mechanism. This  
vulnerability allows an attacker to craft links that contain redirects  
to malicious sites in the ReturnURL parameter.   
  
The exploitation technique detailed in this document bypasses the  
CrossAppRedirects restriction and was successfully performed on  
applications that do not have EnableCrossAppRedirects in the web.config  
(false by default), or have the EnableCrossAppRedirects attribute  
explicitly set to false in web.config.  
  
A friendly formatted version of this advisory is available at:  
http://www.seekersec.com/advisories/AdvMS20120216.html   
  
============  
II. Details  
============  
  
The .NET Forms Authentication Redirect From Login mechanism is  
vulnerable to insecure redirect attacks.   
  
http://<myApplication>/Login.aspx?ReturnURL=  
By sending specially crafted input containing a backslash ("\") at the  
end of the URL as the value of the ReturnURL parameter it is possible to  
bypass the CrossAppRedirects restrictions and embed links that lead to  
sites outside the current application.   
  
Once a victim follows the specially crafted link he indeed arrives at  
the login screen of the vulnerable application. Once he submits his  
username and password and performs a successful login instead of being  
redirected to the URL in the DefaultURL property he will be redirected  
to the URL in the value of the ReturnURL parameter.   
  
To reproduce a simple Login page can be created, for example Login.aspx,  
and the following in web.config:  
  
<authentication mode="Forms">  
<forms name=".ASPXFORMSDEMO" loginUrl="Login.aspx"   
protection="All" path="/" timeout="30" />  
</authentication>  
  
===========  
IV. Exploit  
===========  
  
An attacker can embed any link that terminates with the "\" (backslash)  
character, for example:  
http://<myApplication>/Login.aspx?ReturnURL=http://www.seekersec.com\   
  
==================  
V. Vendor Response  
==================  
  
Microsoft has released a fix for this vulnerability, see  
http://technet.microsoft.com/en-us/security/bulletin/MS11-100 for  
further information   
  
=====================  
VII. Affected Systems  
=====================  
  
.NET frameworks 2.0, 3.5 and 4.0 were tested and found vulnerable to  
this flaw in simple login that has Form Authentication web.config  
configuration with default parameters.   
  
============  
VIII. Credit  
============  
The vulnerability was discovered by Irene Abezgauz, Seeker Security  
  
--------------------  
Irene Abezgauz  
Product Manager  
Seeker Security  
www.seekersec.com   
E-Mail: [email protected]  
  
`