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: www.microsoft.com/net
found: 2011-10-02
by: K. Gudinavicius / SEC Consult Vulnerability Lab
m. / SEC Consult Vulnerability Lab
=======================================================================
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: 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:
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:
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:
-------------
www.sec-consult.com/en/advisories.html
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
SEC Consult Unternehmensberatung GmbH
Office Vienna
Mooslackengasse 17
A-1190 Vienna
Austria
Tel.: +43 / 1 / 890 30 43 - 0
Fax.: +43 / 1 / 890 30 43 - 25
Mail: research at sec-consult dot com
EOF K. Gudinavicius, J. Greil / @2011