Update: CNET has reported that Microsoft said the flaw does not affect the final version of Windows 7.

Microsoft is looking into a zero-day vulnerability today, which reportedly affects systems running Windows Vista and 7. Researcher Laurent Gaffie said that a hacker could exploit the flaw on Windows 7 to cause a critical system error. The flaw lies in a Server Message Block 2 (SMB2) driver.

Gaffie said in a blog post yesterday, "SRV2.SYS fails to handle malformed SMB headers for the NEGOTIATE PROTOCOL REQUEST functionality." People who have commented on his blog post are reporting that the exploit can not only lead to denial of service, but also remote code execution.

Gaffie has contacted Microsoft, and it has since responded by saying that it is investigating the issue but that it is "unaware of any attacks trying to use the claimed vulnerability or of customer impact." The H has successfully tested the proof-of-concept code, which caused a reboot on Vista -- but did not work on Windows 7.