A vulnerability in Windows 2000's file sharing protocol that Symantec rang the alarm on Thursday morning is actually not a new bug, but is a flaw patched more than a year ago, Microsoft claimed later in the day.

Early Thursday, Symantec warned its enterprise customers of a coming exploit of a "zero-day," or unpatched, vulnerability in Windows 2000's SMB (Server Message Block) protocol. Symantec based its warning on a posting to the Dailydave security mailing list, where Immunity Security, the makers of the CANVAS exploit-creation platform, said that it would release details of the exploit next month.

"Immunity is considered to be a reliable source and we are of the opinion that this information should be treated as fact," read Symantec's warning.

Within hours, Microsoft countered Symantec's alert with a blog entry from the Microsoft Security Response Center (MSRC) team. "We've investigated this claim and found the vulnerability being discussed is fixed by MS05-011, a security update released almost 16 months ago," wrote Stephen Toulouse, a MSRC program manager.

"What is new is that someone reportedly has found a different way to exploit the vulnerability. But if you have the update, you're protected," added Toulouse.

Windows 2000 was been patched several times against SMB bugs, including the February 2005 fix of MS05-011 and another in June of that year.

Symantec's gaffe was its second embarrassment of the day. Also on Thursday, eEye Digital Security warned that Symantec's Anti-Virus Corporate Edition 10.0, as well as some of its consumer-grade anti-virus software, could be compromised remotely by a worm without any user interaction.