Microsoft late last week said it won't patch Windows XP for a pair of bugs it quashed Sept. 8 in Vista, Windows Server 2003 and Windows Server 2008.

The news adds Windows XP Service Pack 2 (SP2) and SP3 to the no-patch list that previously included only Windows 2000 Server SP4.

"We're talking about code that is 12 to 15 years old in its origin, so backporting that level of code is essentially not feasible," said security program manager Adrian Stone during Microsoft's monthly post-patch Webcast , referring to Windows 2000 and XP.

"An update for Windows XP will not be made available," Stone and fellow program manager Jerry Bryant said during the Q&A portion of the Webcast ( transcript here ).

Last Tuesday, Microsoft said that it wasn't patching Windows 2000 because creating a fix was "infeasible."

The bugs in question are in Windows' implementation of TCP/IP, the Web's default suite of connection protocols. All three of the vulnerabilities highlighted in the MS09-048 update were patched in Vista and Server 2008. Only two of the trio affect Windows Server 2000 and Windows XP, Microsoft said in the accompanying advisory, which was refreshed on Thursday.

In the revised advisory, Microsoft explained why it won't patch Windows XP, the world's most popular operating system . "By default, Windows XP SP2, Windows XP SP3 and Windows XP Professional x64 Edition SP2 do not have a listening service configured in the client firewall and are therefore not affected by this vulnerability," the company said. "Windows XP SP2 and later operating systems include a stateful host firewall that provides protection for computers against incoming traffic from the Internet or from neighboring network devices on a private network."

Although the two bugs can be exploited on Windows 2000 and XP, Microsoft downplayed their impact. "A system would become unresponsive due to memory consumption ... [but] a successful attack requires a sustained flood of specially crafted TCP packets, and the system will recover once the flood ceases."

Microsoft rated the vulnerabilities on Windows 2000 and XP as "important" on Windows 2000, and as "low" on XP. The company uses a four-step scoring system, where "low" is the least-dangerous threat, followed in ascending order by "moderate," "important" and "critical."

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