Microsoft's latest Patch Tuesday update brought six patches, three of which Redmond described as critical fixes.

The critical update covers flaws in Excel, Windows Active Directory, and .NET Framework. All create a possible means for hackers to inject hostile code onto vulnerable systems (remote code execution). Separate security bugs in Internet Information Server (Microsoft's web server software) and Microsoft Office Publisher also carry the same risk but earn a lower classification of "important" from Redmond. Microsoft's security gnomes have also addressed a "moderate" security bug in Windows firewall that creates an information disclosure risk.

Microsoft summary can be found here. The SANS Institute's take on "Black Tuesday" can be found here. It reckons that the IIS, Office Publisher and Windows firewall bug flaws merit a higher security classification than assigned to them by Redmond, depending on a user's systems.

None of the flaws patched on Tuesday are the subject of active hacker exploits but security researchers highlight the .NET Framework flaw as potentially the most troublesome.

"The patch for the .NET Framework is the most important patch this month. It has the potential to affect every application running on every operating system that Microsoft is actively supporting today," said Alan Bentley, md of patching and vulnerability assessment firm PatchLink. The Excel vulnerability, since it lends itself to the creation of malware targeting unpatched vulnerabilities, is also a serious concern. he added. ®