Windows 7 hasn't even been on the street for a month, yet one hacker has already found what he claims is a zero-day (define) vulnerability -- albeit so far the result of an attack is simply to crash the new system.

Hacker Laurent Gaffié, who last August found a problem in the "release candidate" (RC) version of Microsoft (NASDAQ: MSFT) Windows 7, announced his find on Wednesday. He also published proof-of-concept code to cause such crashes.

After some controversy last summer, Microsoft said that the bug Gaffié found that time affected some late, pre-release code, but that it did not impact the final released Windows 7 code.

The company subsequently patched the hole in affected operating systems in October.

Microsoft did patch a pair of "important" security holes in Windows 7 in its October Patch Tuesday bug fix drop. However, they were not rated rated as "critical" -- the top tier of Microsoft's four-tier bug severity scale.

As last time, Gaffié publicized his latest find on his own blog, as well as the Full Disclosure blog site.

The bug that Gaffié found this time is related to the same problem he found last summer. It resides in a networking protocol called System Message Block or SMB. While the hole he found in August was only related to version two -- known as SMB2 -- the new hole affects both SMB1 and SMB2.

"Whatever your firewall is set to, you can get remotely smashed via IE [Internet Explorer] or even via some broadcasting [NetBIOS Name Service] tricks (no user interaction)," Gaffié said in his blog post.

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The result, he said, is to cause a software kernel crash in either Windows 7 or in Windows Server 2008 Release 2 (R2). The crash causes a denial-of-service but doesn't compromise the user's PC -- at least not so far.

So far, Microsoft has said only that it's looking into the issue.

"Microsoft is investigating new public claims of a possible Denial-of-Service vulnerability in Windows Server Message Block. We’re currently unaware of any attacks trying to use the claimed vulnerability or of customer impact," Christopher Budd, security response communications lead at Microsoft, said in an e-mail.

"Once we’re done investigating, we will take appropriate action to help protect customers," Budd added.

Microsoft only earlier this week patched several critical bugs when it released its November Patch Tuesday fixes.