The flaw in the Windows Server Message Block (SMB) network and file-sharing protocol was disclosed Monday by someone identified only as 'Cupidon-3005' on the Full Disclosure security mailing list. Cupidon-3005 posted proof-of-concept code to the list.
French and Danish researchers later said hackers might be able to exploit the bug to hijack Windows PCs. However, Microsoft said that wasn't so.
"Based on our initial investigation this vulnerability cannot be leveraged for remote code execution (RCE) on 32bit platforms," said Jerry Bryant, a general manager in the Microsoft Security Response Center (MSRC). "We are still investigating the possibility of code execution on 64bit platforms, but so far have not found a likely scenario that would result in reliable code execution."
A successful attack that exploits the SMB bug would instead result in a 'denial of service', said Bryant, using the term that describes a Window crash that would require rebooting the PC. Windows crashes often inform users of the dire situation with the infamous 'Blue Screen of Death'.
In a explanatory blog post , MSRC engineer Mark Wodrich echoed Bryant's take on the likelihood of remote code execution, saying that it was impossible on a 32bit version of Windows due to memory limitations, and feasible on 64bit Windows only if more than 8GB of memory was present.
Even then, said Wodrich: "We feel that triggering any such timing condition reliably will be very difficult".
HD Moore, chief security at Rapid7 and the creator of the open-source Metasploit penetration toolkit, chimed in as well.
"We haven't seen any solid examples of code execution yet, even if it turns out to be possible," Moore said. He added that a Metasploit researcher was also examining the bug and Cupidon-3005's published attack code to see if a reliable Metasploit exploit module could be crafted.
All versions of Windows contain the bug, said Wodrich, but servers running as the Primary Domain Controller (PDC) - the system that controls the network domain - are most vulnerable.
Microsoft pegged the vulnerability's Exploitability Index ranking at '3', meaning the company doesn't expect reliable attack code will appear in the next 30 days.
Some researchers chastised Microsoft for downplaying the threat.
"Microsoft is now calling any hard-to-exploit vulnerability (e.g. SMB) a 'Denial of Service'! What's next?" asked French security firm Vupen in a tweet.
"They've been doing this forever, MSRC is about managing PR incidents, not improving security," said Tavis Ormandy in a reply to Vupen's tweet.
Ormandy, a Google security engineer, has butted heads with Microsoft before - most notably last summer, when he released exploit code for a bug in Windows' Help and Support Center after he said Microsoft refused to set a patch deadline.
Microsoft's Bryant said the MSRC researchers are will investigating the bug, and would issue a patch or a workaround to protect users.
Although the company's next regularly-scheduled Patch Tuesday is three weeks away, it's unlikely a fix will be delivered then unless a large number of in-the-wild attacks exploiting the vulnerability appear.