Microsoft has released its monthly set of security patches, fixing critical flaws in the Windows desktop.

The December updates released Tuesday include critical fixes for Internet Explorer, DirectX and the Windows Media Format technology. Security experts say that the most important of these updates is the Internet Explorer patch, because it fixes four separate bugs in the browser. One of these flaws, relating to the way the browser renders dynamic HTML (DHTML) pages, has been exploited in online attacks, Microsoft said.

All of the browser vulnerabilities in this update are rated critical -- Microsoft's most serious rating -- for IE 7 users running on the latest version of Windows XP, Microsoft said.

Though Microsoft has assigned it the less-grave rating of "important," a patch for Macrovision copy-protection software that ships with Windows should also be given priority, security experts say. That's because criminals have already leveraged this bug in online attacks.

The Macrovision issue first cropped up in mid-October, when Symantec spotted attackers exploiting the flaw, but Microsoft was not able to ship a fix for the problem in last month's security updates, released Nov. 13. The flaw could be used by attackers to allow their software to run at a higher level of privilege within the operating system.

The flaw lies in the secdrv.sys driver that is used by Macrovision's SafeDisc system, which ships with Windows XP and Windows Server 2003.

Microsoft also issued important updates for the Windows Vista Server Message Block (SMB) version 2 filesharing protocol, the Vista kernel, and the Message Queuing Service in XP and Windows 2000.

Microsoft did not fix a recently publicized flaw in the way its Windows operating system looks up other computers on the Internet. This bug, which was publicized at a hacker conference in New Zealand, has to do with the way Windows systems look for DNS (Directory Name Service) information under certain configurations.

Interestingly, the Vista SMB flaw lies in a feature that allows senders to digitally sign SMB data in order to confirm that it is legitimate. Because the signing feature is not properly implemented, however, "an attacker could modify SMBv2 packets and impersonate a trusted source to perform malicious operations," Microsoft said.

"It's a security vulnerability in a security feature," said Eric Schultze, chief technology officer of Shavlik Technologies, via instant message. "SMB version 2 was built for Vista and Windows Server 2008, so it should have been vetted in the code design process. But it obviously slipped through."

In all, seven sets of patches were released Tuesday, fixing 11 vulnerabilities.

Though Microsoft has made much of its efforts to develop more secure software, the company ended 2007 with about the same number of security updates that it had in the year before, according to security vendor Kaspersky Lab. "The situation in 2007 hasn't changed noticeably from 2006," wrote David Emm, a senior technology consultant with Kaspersky, in a blog post. "Last year there were 49 critical, 23 important, and 5 moderate updates. 2007 brought very slightly fewer patches, with 43 critical, 24 important, and 2 moderate fixes."