Flaw In Microsoft's Hypervisor Lets Attackers Bypass DEP, ASLR

Core Security Technologies has discovered a flaw in Microsoft's Virtual PC hypervisor that can be used by an attacker to cheat built-in, advanced security features in Windows.

The flaw in the memory management function of the hypervisor affects Microsoft Virtual PC 2007, Virtual PC 2007 SP1, Windows Virtual PC, Microsoft Virtual Server 2005, and the XP Mode feature in Windows 7. Microsoft's Hyper-V technology is not affected by the vulnerability.

Core first reported its discovery of the vulnerability to Microsoft in August. Microsoft will fix the issue in future updates to the products, according to Core. For now, users should run mission-critical applications on native hardware and software platforms, or employ virtualization software not affected by the flaw.

Microsoft says the attack is based on using existing vulnerabilities rather than an actual vulnerability itself: "It does this by rendering a number of protection mechanisms that are present in the Windows kernel less effective inside a virtual machine as opposed to a physical Windows machine. An attacker would need to abuse an already present vulnerability in order to leverage this technique," a Microsoft spokesperson says.

And an attacker would not be able to take over an entire host machine running multiple virtual machines, the spokesperson notes. "An attacker can only exploit a vulnerable application running inside the guest virtual machine on Windows XP, rather than Windows 7," according to Microsoft.

But according to Core, the danger of the vulnerability is in its ability to bypass Windows' Data Execution Prevention (DEP), Safe Exception Handlers (SafeSEH), and Address Space Layout Randomization (ASLR) features in some versions of Windows. So applications with nonexploitable bugs in nonvirtualized environments could then be exploited when run in a guest operating system in the Virtual PC, says Ivan Arce, CTO of Core.

"This makes all kinds of exploits easier to exploit," Arce says. "DEP and ASLR are ineffective," he says.

This is an example of a future trend in attacks, he says, where attackers writing exploits for code execution will look for ways to circumvent memory address space. While security features like DEP and ASLR have improved Windows security overall, he says, adding virtualization can introduce new vulnerabilities.

"There may be issues in virtualization that will impact the rest of the OS or infrastructure, and this is an example of that," Arce says. "I think this is going to be an ongoing trend in the future, where [attackers] are not just attempting to circumvent security protections, but using virtualization to find different problems and exploit them."

Meanwhile, the memory management bug in the Virtual Machine Monitor lets an attacker access so-called "high" memory above 2 gigabytes and gain read or read/write privileges from apps running in the guest OS, according to Core.

Core also released proof-of-concept code demonstrating the flaw and how it could be exploited.

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