"This isn't a straight ASLR bypass as it only works under certain conditions," said Bryant. "An attacker would have to use this in conjunction with an unpatched vulnerability in order to exploit a system." In the same e-mail, Bryant declined to label the bug as a security vulnerability. "This is not a vulnerability but a mitigation bypass technique," he said.
Last month, someone identified only as "fl0 fl0w" posted exploit code for a flaw in an important code library used to develop third-party software using Microsoft's flagship Visual Studio software.
The bug in Microsoft Foundation Classes (MFC), a set of coding libraries that lets developers access Windows APIs (application programming interfaces) when working in C++, can be exploited through some third-party software written with Visual Studio. fl0 fl0w said his attack code can compromise a Windows PC via PowerZip, a low-priced archiving utility.
Microsoft said that its preliminary investigation showed only Windows 2000 and XP were vulnerable to the MFC attack. "We are investigating reports ... [and] will update when we have more information," the company said on its official security Twitter account Monday .
The four newest zero-day reports aren't the only headaches for Microsoft's security engineers: They still have not patched the critical Windows flaw that Tavis Ormandy publicly disclosed last month after Microsoft wouldn't commit to a patching deadline .
Ormandy, who works for Google's security team, has been at the center of a debate between researchers over his decision to go public. The Microsoft-Spurned Researcher Collective was formed as a reaction to Microsoft explicitly linking Ormandy and his employer.
His vulnerability has been actively exploited by hackers since June 15.
The next scheduled Patch Tuesday for Microsoft is July 13. The company has been tight-lipped about whether it will patch Ormandy's vulnerability, but based on past practice, it's highly unlikely that the Microsoft could assemble and test fixes for the other recent zero-day bugs in time to make next week's deadline.