SEC Consult Vulnerability Lab Security Advisory < 20111230-0 >
=======================================================================
              title: Microsoft ASP.NET Forms Authentication Bypass
            product: Microsoft .NET Framework
 vulnerable version: Microsoft .NET Framework Version:4.0.30319; 
                     ASP.NET Version:4.0.30319.237 and below
      fixed version: MS11-100
                CVE: CVE-2011-3416
             impact: critical
           homepage: http://www.microsoft.com/net
              found: 2011-10-02
                 by: K. Gudinavicius / SEC Consult Vulnerability Lab 
                     m. / SEC Consult Vulnerability Lab
                     https://www.sec-consult.com 
=======================================================================

Vendor description:
-------------------
".NET is an integral part of many applications running on Windows and
provides common functionality for those applications to run. This
download is for people who need .NET to run an application on their
computer. For developers, the .NET Framework provides a comprehensive
and consistent programming model for building applications that have
visually stunning user experiences and seamless and secure
communication."

Source: http://www.microsoft.com/net



Vulnerability overview/description:
-----------------------------------
The null byte termination vulnerability exists in the
CopyStringToUnAlingnedBuffer() function of the webengine4.dll library
used by the .NET framework. The unicode string length is determined
using the lstrlenW function. The lstrlenW function returns the length
of the string, in characters not including the terminating null
character. If the unicode string containing a null byte is passed, its
length is incorrectly calculated, so only characters before the null
byte are copied into the buffer.

This vulnerability can be leveraged into an authentication bypass
vulnerability. Microsoft ASP.NET membership system depends on the
FormsAuthentication.SetAuthCookie(username, false) method for certain
functionality. By exploiting this vulnerability an attacker is able to
log on as a different existing user with all the privileges of the
targeted user (e.g. admin).



Proof of concept:
-----------------

Detailed exploit information and source code references have been
removed from this advisory.

An attacker is able to bypass authentication in certain functionality
using null bytes and log on as another user, e.g. admin.


Vulnerable / tested versions:
-----------------------------
The vulnerability has been verified to exist in Microsoft .NET Framework
Version:4.0.30319; ASP.NET Version:4.0.30319.237, which was the most
recent version at the time of discovery.

More information regarding affected versions is available within the
advisory of Microsoft:
http://technet.microsoft.com/en-us/security/bulletin/ms11-100


Vendor contact timeline:
------------------------
2011-10-07: Contacted vendor through secure@microsoft.com
2011-10-07: Vendor response, MSRC 11838
2011-10-14: Contacted MSRC asking for status
2011-10-15: Answer from case manager: the vulnerability will be
            addressed through a security bulletin, a timeframe is
            unknown.
2011-11-23: Contacted MSRC asking for status
2011-11-23: Answer from case manager: a release date of update is
            unknown, best guess would be a month before or after the
            March (2012) update cycle
2011-12-29: Microsoft publishes out-of-band security patch MS11-100
            which also addresses this vulnerability
2011-12-30: SEC Consult releases redacted version of advisory due to
            criticality of this issue

SEC Consult will release a more detailed advisory at a later date.



Solution:
---------
Immediately apply the MS11-100 patch:
http://technet.microsoft.com/en-us/security/bulletin/ms11-100


Workaround:
-----------
In .NET 4.0 the vulnerability can be mitigated by setting the
ticketCompatibilityMode attribute in the application or global
web.config file like this:

<system.web>
  <authentication mode="Forms">
    <forms ticketCompatibilityMode="Framework40" />
  </authentication>
</system.web>



Advisory URL:
-------------
https://www.sec-consult.com/en/advisories.html


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Mail: research at sec-consult dot com
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EOF K. Gudinavicius, J. Greil / @2011