Microsoft has patched 28 vulnerabilities, nearly all of them marked "critical," in the biggest update since it switched to a regular monthly update schedule more than five years ago.

Of the 28 bugs quashed today, Microsoft ranked 23 of them critical, the top rating in its four-step scoring system. Of the five others, three were judged "important," the next step down, and two were pegged "moderate." The patches were issued in eight updates for Windows, Internet Explorer (IE), Office, SharePoint, Windows Media, and the company's most popular development tools, Visual Basic and Visual Studio.

Researchers agreed that one of the Windows updates should be tops on everyone's to-do list. "There are a few that will stick out for a lot of people," said Andrew Storms, director of security operations at nCircle Network Security. "The GDI is one."

MS08-071, which contains two separate vulnerabilities, both critical, updates the Graphics Device Interface (GDI), the core graphics rendering component of Windows. GDI has been repeatedly patched by Microsoft, most recently in September.

"This looks very similar to MS08-021," said Storms, referring to an April update that patched two more GDI bugs. Like that earlier fix, as well as the one in September, hackers could exploit the vulnerabilities by duping users into opening or viewing malicious WMF (Windows Metafile) images.

"[MS08-071] is something similar to what we saw with WMF files once before this year, and once last year, too," said Amol Sarwate, manager of Qualys's vulnerability lab. "It's in the core kernel, it's always there, it's in all versions of Windows and the attack vector is pretty high." Like Storms, Sarwate put the update at the top of his list.

The long-running patch job on GDI will, said Storms, inevitably prompt some to ask whether Microsoft's vaunted SDL (Security Development Lifecycle) process, under which it scrutinises code as its written for bugs, really works. "Is SDL functioning? I don't know," Storms admitted. "Without seeing the code analysis, it's difficult to presume it's not."

"Yes, I think that's a fair question," said Wolfgang Kandek, the chief technology officer for Qualys. "But is it realistic to expect Microsoft to find everything? No, it's not."

Storms said the IE update, MS08-073, would be his next highest update priority, simply because of the number of vulnerabilities it fixes - four, all critical - and because of the dominance of Microsoft's browser. After that, it gets murkier. "GDI and IE are certainly top of the list, but beyond that it's a toss-up," he said. "It's going to be difficult for people in the trenches to understand what to go after the first and second."

Qualys' Sarwarte and Kandek, meanwhile, staked out MS08-070 as the second-most-interesting update among today's eight. "This is a far-reaching vulnerability," said Kandek, who noted that while end users won't be installing this update for Visual Basic, it can potentially affect anyone who browses the Internet with IE.

This month's eight security updates can be downloaded and installed via the Microsoft Update and Windows Update services, as well as through Windows Server Update Services.