Microsoft Tuesday issued six critical patches, one less than expected, covering Windows, Office, Internet Explorer and Windows Media Player. Five other patches rated as important were delivered as part of Microsoft's monthly Patch Tuesday release.

The list of critical patches had so far been light this summer. There were three in June and none in July. Microsoft has issued a total of 51 patches so far this year.

Office was perhaps the hardest hit this month with Word, Excel, Access and PowerPoint all having vulnerabilities. Internet Explorer had one patch but six vulnerabilities. The patches mostly were centered on the client side rather than the server side.

August's release addressed 26 separate vulnerabilities in the 11 patches, the largest number of patches in one month since the 11 released in February.

Four of the August vulnerabilities were classified as zero day: MS08-041 (Access), MS08-042 (Word), MS08-045 (Internet Explorer), MS08-050 (Windows Messenger).

"Even though 50 is rated only as important, it is categorized as information theft," says Amol Sarwate, manager of vulnerabilities research lab at Qualys. "An attacker can steal the user's Messenger ID and they can use it to call people for audio and video conferences and to see all the user's chat information."

Christopher Budd, security response communications lead for Microsoft, said in a statement that the planned seventh critical patch, which Microsoft announced last week, was held back due to quality issues. The statement did not say when that patch would be released, but it is likely to find its way into the September release unless the vulnerability begins to be actively exploited.

The patch for Internet Explorer (MS08-045) addresses a combination of five privately reported vulnerabilities and one publicly disclosed vulnerability. The vulnerabilities affect Internet Explorer version 5.01, 6.0, 6.0 (SP1), and 7 on various versions of Windows including Windows 2000, XP, XP Pro, Vista, and Windows Server 2003 and 2008.

Four of the vulnerabilities deal with HTML, points out Don Leatham, director of solutions and strategy at Lumension Security. "All four can provide a hacker with remote code execution across IE 6 and 7 and across almost all versions of Windows. Every single Web page in the world has some level of HTML and so we think this is one people need to get ahead of. This is going to be a playground for hackers."