Microsoft has produced its monthly security updates, patching critical flaws in Internet Explorer, Windows Media Format and Visual Studio 2005.

The seven security patches cover 11 bugs, including two in Windows Media Player. However, no fixes were provided for two Microsoft World flaws that have been used in a number of attacks in the past week.

Microsoft had said it would release only six patches on Tuesday, but the company added the Windows Media Format update at the last minute, after reports of attacks based on this vulnerability began surfacing.

In late November, security vendors warned that a buffer overflow error could occur when the Windows Media Player processed ".asx" media files, meaning that users would first need to be tricked into opening a malicious media file for the attack to work. In its update on Patch Tuesday, Microsoft also patched a similar bug in the way the media player processes ".asf" (Advanced Systems Format) files.

The Internet Explorer patch fixes four bugs. It is also rated critical, and is noteworthy because some of these bugs will probably begin to be exploited by hackers by week's end, said Gunter Ollmann, director of IBM.'s Internet Security System's X-Force threat analysis service.

Enterprise administrators should also pay close attention to an SNMP (Simple Network Monitoring Protocol) patch issued Tuesday, Ollmann said.

Microsoft has rated this patch as "important", rather than critical because SNMP is normally blocked at the firewall and turned off by default on Windows systems. However, it is widely deployed as part of the network monitoring infrastructure in the enterprise, and is often used on critical servers, Ollmann said.

Ollmann believes this SNMP patch is the most important update for enterprise customers. "Since the service is widely deployed in the enterprise and since it's commonly deployed on servers, we think this would be an important attack vector for enterprises," he said.

The remaining updates include a "critical" fix for Visual Studio 2005, and "important" updates for Windows and Outlook Express, Microsoft said. The company's next set of security updates are due 9 January.