With its massive security update last month, Microsoft Corp. marked the end of the sixth year since it moved to a monthly patch schedule. An informal count of releases in Microsoft's bulletin archives shows that the company has released about 400 security bulletins since October 2003.

The bulletins have addressed about 745 vulnerabilities across almost every Microsoft product. More than half of the bulletins (230) have addressed vulnerabilities that Microsoft called "critical," meaning they would allow attackers to take full administrative control of a system from a remote location.

The total number of flaws disclosed and patched by the software maker so far this year stands at about 160; Microsoft reported 155 for all of 2008. The tally for the past two years is more than double the number of flaws disclosed in 2004 and 2005, the first two full years of Patch Tuesdays.

The last time Microsoft didn't have any patches on a Patch Tuesday was March 2007. In the past six years, Microsoft has had just four patch-free months -- two of which were in 2005. In contrast, the company issued patches for 10 or more vulnerabilities on more than 20 occasions and patches for 20 or more flaws in a single month on about 10 occasions.

This version of this story originally appeared in Computerworld 's print edition. It's an edited version of an article that first ran on Computerworld.com.