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Microsoft issued two emergency updates on Tuesday to fix critical security bugs that leave users of Internet Explorer and an untold number of third-party applications vulnerable to remote attacks that completely commandeer their computers.

Most of the vulnerabilities are located in Microsoft's ATL, or Active Template Library, which developers from Redmond and elsewhere use to write Component Object Model code, including ActiveX controls that are frequently targeted by attackers. Applications that have drawn on the buggy library may be vulnerable to attacks that allow the remote execution of malware and the interception of sensitive user information.

The first patch, referred to as MS09-035, repairs three vulnerabilities in the ATL contained in Microsoft's Visual Studio suite. Developers who have used Visual Studio to create controls may need to recompile their programs after patching their systems. Third-party developers wondering if a given application they've written is vulnerable can visit this page to find out more.

Microsoft doesn't know how many third-party applications are affected, but believes the "vast majority" are not, said Mike Reavey, director of the Microsoft Security Response Center.

The second patch is referred to as MS09-034. It prevents any components or controls built using the buggy ATL from being exploited in IE. It also fixes three unrelated vulnerabilities in the Microsoft browser that could also lead to remote code execution.

As previously reported, one of the ATL bugs being squashed allowed attackers to bypass so-called kill-bit protections, which are used to seal off ActiveX controls that are later determined to be insecure or otherwise unreliable. The bug is significant because it has the effect of resurrecting vulnerabilities that were patched long ago.

"If you have a machine that's not been updated with the update we released today, then any vulnerable control on your system could expose you to risk, like any scenario where you have a vulnerable component on your machine," Reavey told The Register. "If you don't have the updates we released today then you're exposed to old vulnerabilities in ATL that are mitigated through IE today."

The ATL vulnerabilities are also noteworthy because they have the ability to pollute large bases of code that flow out of Visual Studio's ATL. The bug is dubbed the "ATL COM initialization vulnerability and given a designation of CVE-2009-2493.

Microsoft fixed two other security flaws in the ATL. One is dubbed ATL Null String Vulnerability and the other ATL uninitialized object vulnerability.

"This is an issue in a library, and when they happen throughout the industry, they're challenging," Reavey said. "The biggest theme here is collaboration."

This is only the ninth time Microsoft has issued security updates outside of its normal schedule. It comes as thousands of hackers and security professionals convene in Las Vegas for the Black Hat and Defcon conferences.

Already, Adobe has chimed in to say that its both its Flash and Shockwave media player applications use vulnerable versions of the ATL, but only versions of those programs that use IE are affected. The company plans to issue fixes. In the interim, users should be protected by installing the patch for MS09-034.

While Microsoft's swift action is commendable, PC users should remember that Tuesday's updates are only the beginning. If you're using an ActiveX control that loads in an application other than IE, there's still the very real possibility that it has been poisoned by Microsoft's ATL and isn't fixed by these updates, said Ryan Smith, one of the researchers who discovered the killbit-override bug.

Smith and fellow researchers Mark Dowd and David Dewey plan to discuss one of the issue during this talk at Black Hat. ®

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