Microsoft ASP.NET Forms Authentication Bypass

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


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."






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:



Vendor contact timeline:


2011-10-07: Contacted vendor through

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.






Immediately apply the MS11-100 patch:





In .NET 4.0 the vulnerability can be mitigated by setting the

ticketCompatibilityMode attribute in the application or global web.config

file like this:



<authentication mode="Forms">

<forms ticketCompatibilityMode="Framework40" />






Advisory URL:





SEC Consult Unternehmensberatung GmbH


Office Vienna

Mooslackengasse 17

A-1190 Vienna



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