Microsoft on Thursday released its first and only out-of-band patch for 2011. But it's a critical one. MS11-100 addresses a vulnerability in ASP.NET that could allow a denial of service attack.
Dave Forstrom, Microsoft's director of trustworthy computing, said the security update is rated critical. The fix addresses a remote unauthenticated denial of service issue in ASP.NET versions 1.1 and above on all supported versions of .NET Framework. But Forstrom warned that Microsoft is not the only vendor that is vulnerable.
"Of note, the new method of hash collision attacks used to exploit this vulnerability is an industrywide issue affecting various Web platforms, including ASP.NET," Forstrom said. "While we have seen no attacks attempting to exploit this vulnerability, we encourage affected customers to test and deploy the update as soon as possible. Consumers are not vulnerable unless they are running a Web server from their computer."
Executing Remote Commands
In its security bulletin, Microsoft explained if an unauthenticated attacker sends a specially crafted Web request to a Web site targeted for take down it could open a back door. An attacker who successfully exploits this vulnerability could manipulate the ASP.NET site, executing remote commands at will.
However, in order to exploit this vulnerability an attacker must be able to register an account on the ASP.NET site, and must know an existing user name. Microsoft said customers who have not enabled automatic updating need to check for updates and install this update manually.
The out-of-band patch tackles four issues: CVE-2011-3416, an ASP.NET forms authentication bypass issue rated as critical; CVE-2011-3414, a hash table collision DoS issue rated as important; CVE-2011-3417, an ASP.NET ticket caching vulnerability rated important; and CVE-2011-3415, an insecure redirect vulnerability rated moderate.
How the Fix Works
"Microsoft tested and finished MS11-100 in record time, taking about 30 days for the process of integrating this new vulnerability with the fix that was already scheduled for January 2012," said Wolfgang Kandek, CTO of Qualys. "We consider Microsoft's reaction and implementation speed outstanding, as they were only notified at the tail end of the German security researchers' work."
Qualys is tracking how the other projects and vendors affected, including PHP, Oracle, Python, Ruby and others, are rolling out their patches.
"The bulletin fixes the DoS attack vector by providing a limit to the number of variables that can be submitted for a single HTTP POST request," Kandek said. "The default limit is 500 which should be enough for normal Web applications, but still low enough to neutralize the attack as described by the security researchers in Germany. This addresses the most obvious attack method immediately and leaves the reimplementation of the hash function for a future update."