Microsoft Corp.'s massive security update yesterday marked the completion of the sixth year of the company's move to a monthly patch release schedule.

Since moving to a monthly schedule in October 2003, Microsoft has released about 400 security bulletins based on an informal count of releases in its bulletin archives . The bulletins address about 745 vulnerabilities across almost every Microsoft product.

About 230, or more than half, of the bulletins addressed security vulnerabilities that were described by Microsoft as "critical." It's definition that Microsoft typically uses for vulnerabilities that allow attackers to take full administrative control of a system from a remote location.

More vulnerabilities are being discovered in Microsoft products these days than when the company first moved to a monthly patch schedule. The total number of flaws disclosed and patched by the software maker so far this year stands at around 160, more than the 155 or so that Microsoft reported for all of 2008. The number of flaws reported in Microsoft products over the last 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 did not release any patches on a Patch Tuesday was March 2007, more than 30 months ago. In the past six years, Microsoft has had just four patch-free months -- two of them in 2005. In contrast, the company has 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, including yesterday.

The increase in the number of flaws being discovered comes at a time when attackers are getting much faster at exploiting them. A survey by security vendor Qualys earlier this year showed that 80% of vulnerability exploits are available within 10 days of the vulnerability's disclosure. Nearly 50% of the vulnerabilities patched by Microsoft in its security updates for April this year already had known exploits by the time the patches were available .

The numbers highlight Microsoft's continuing challenges on the security front, said David Rice, president of the Monterey Group, a security consultancy in Monterey, Calif. and author of Geekonomics: The Real Cost of Insecurity Software . But it is important to keep them in perspective, he added.

Other major vendors, such as Oracle Corp. and Apple Inc., have also announced large numbers of vulnerabilities over the past few years, Rice said. But neither of these two vendors has invested anywhere near the money and resources that Microsoft has spent on security over the past several years, largely because of the lack of incentive for them to do so, he said. Unlike other sectors, such as the automobile industry, where a vehicle's safety rating has an impact on product sales, software vendors have not been penalized by consumers for buggy software. So many have chosen not to invest in doing a better job of securing their products, he said.

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