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Computerworld - Editor's note: This story has been updated with corrected information about the number of bugs Microsoft did not patch.

Microsoft took the unusual step today and skipped patching two of the vulnerabilities addressed in its monthly security update, saying that crafting a fix was "infeasible."

The omission leaves users running Windows 2000 Server Service Pack 4 (SP4) vulnerable to attack.

Earlier today, Microsoft delivered five critical updates that patched eight vulnerabilities in Windows, including two that the company won't bother fixing in Windows 2000 Server SP4. The operating system's support doesn't end until July 2010; until then, Microsoft was supposed to provide updates.

"That's really strange," said Jason Miller, Security and Data Team Manager, Shavlik Technologies. "I haven't seen them do this before."

Miller was referring to MS09-048, the security update that patches a pair of vulnerabilities in Windows' implementation of TCP/IP, the Web's default suite of connection protocols. Both of the update's vulnerabilities were patched in Vista and Server 2008, but not for Windows Server SP4.

"Microsoft is not issuing a patch for the vulnerability," continued Miller. "They're stating that creating a patch to address the vulnerability is 'infeasible to build.' With this in mind, a vulnerability that affects Windows 2000 is about to be made known and administrators cannot simply patch their machines."

In the MS09-048 bulletin, Microsoft spelled out why it's not fixing the flaws in Windows 2000 SP4.

"The architecture to properly support TCP/IP protection does not exist on Microsoft Windows 2000 systems, making it infeasible to build the fix. To do so would require re-architecting a very significant amount of the Windows 2000 SP4 operating system, not just the affected component. The product of such a re-architecture effort would be sufficiently incompatible ... that there would be no assurance that applications designed to run on Windows 2000 SP4 would continue to operate on the updated system."

"Oh, my goodness," said Andrew Storms, director of security operations at nCircle Network Security. "[That's] more proof that Windows 2000's network stack was not in good shape," he added, noting that when Windows XP was released, rumors circulated that Microsoft contracted with Cisco Systems to write that operating system's TCP/IP stack.

Miller urged companies to put MS09-048 at the top of their patch to-do list. "This one's critical if you're looking for some business interaction," he said. "Plus, there's no user interaction required for this."

Although Windows 2000 accounts for a very small slice of all Windows machines, client or server, Miller said the flaws -- and the fact that Microsoft's not bothering to fix them -- could be disastrous for some firms.