Microsoft has released its November security updates, fixing a critical Windows bug that has been exploited by online criminals.

Microsoft released just two security updates this month, but security experts say that IT staff will want to install both of them as quickly as possible. The MS07-061 update is particularly critical because the flaw it repairs has been seen in Web-based attack code, said Amol Sarwate, manager of Qualys's vulnerability research lab. "This was a zero day [flaw] that was being used in the wild by hackers," he said.

The flaw has to do with the way Windows passes data between applications using a technology called the URI protocol handler. This is the part of Windows that allows users to launch applications -- an e-mail or instant messaging client, for example -- by clicking on a Web link. Because Windows does not perform all of the security checks necessary, hackers found ways to sneak unauthorized commands into these Web links, and the flaw could be exploited to install unauthorized software on a victim's PC.

This type of flaw lies in both Windows and the programs being launched by the Web link, and Microsoft had initially said that it was up to third-party software developers to fix the issue. It later reversed this position and decided to fix the flaw in Windows as well. These URI protocol handler problems have turned up in Adobe, Firefox, and Outlook Express.

Microsoft was forced to revise its position on the URI bugs after researchers discovered that they were far more problematic than first thought, said Nathan McFeters, a security researcher with Ernst & Young, who has been studying this problem. "I think that early on it wasn't clear that this was an issue," he said. "There's really a handful of issues with this URI use and abuse stuff."

Microsoft's patch for this problem is rated critical for Windows XP and Windows Server 2003 users, but the bug does not affect Windows 2000 or Vista, Microsoft said.

The second vulnerability, rated "important" by Microsoft, has to do with Windows DNS servers, which are used to exchange information about the location of computers on the Internet. Attackers could exploit this flaw to redirect victims to malicious Web sites without their knowledge, something known as a "man in the middle" attack. "All system administrators should look very closely at this vulnerability," Sarwate said. "I would have personally rated it as critical," he said.

Security experts were surprised that Microsoft did not include a patch for a known vulnerability in some Macrovision antipiracy software that has been shipping with Windows for the last few years. Microsoft has said that it plans to patch the problem and that it is aware of "limited attacks" that exploit this vulnerability to get elevated privileges on a victim's machine.

The bug lies in the secdrv.sys driver built by Macrovision that ships with Windows XP, Server 2003, and Vista, but Vista is not vulnerable to the problem, according to Microsoft.

Macrovision has also published a patch for this problem.

Its a "bit worrisome" that Microsoft hasn't pushed out a patch for the bug, given that Macrovision has already made its fix available, said Andrew Storms, director of security operations with nCircle Network Security, via instant message. "However, [it's] understandable that Microsoft would want to run the patch through its QA and software release cycles," he added. "Given the choice between the URI bug and the Macrovision fix, enterprise security operations teams would much rather have the URI fix."

Users of Microsoft's WSUS (Windows Server Update Services) update system had been wondering if they were going to get Tuesday's patches after a Microsoft programming error knocked WSUS administration consoles offline on Sunday and Monday. Microsoft had misnamed an entry in WSUS's database causing the consoles to crash.

The problem was fixed on Monday, said Bobbie Harder, a Microsoft senior program manager, in a blog posting. But WSUS servers that synchronized with Microsoft between 5 p.m. Sunday and 11 a.m. Monday Pacific Time will need to resynchronize to avoid the problem.

Though she had heard of one user who had to manually updated his WSUS server, Tuesday's updates went off without a hitch, said Susan Bradley, a WSUS user who is CTO with Tamiyasu, Smith, Horn and Braun, Accountancy.

This story was updated on November 13, 2007