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A newly discovered critical vulnerability in VMWare’s Windows desktop software could let an attacker break out of the VM environment "barrier" and infiltrate the underlying host system.

The vulnerability, which was found by Core Security Technologies, lets an attacker create or alter executable files on the Windows host OS -- but only if VMware’s Shared Folders feature is enabled and at least one folder on the underlying host system is configured to share files with the VM.

VMware plans to issue a patch within two weeks for the bug, which occurs in Windows host systems running VMware Workstation, VMware Player, and VMware ACE software, according to Jerry Chen, senior director for enterprise desktop VMware. VMware Server is not affected by this vulnerability.

Ivan Arce, CTO of Core Security, says the trouble with virtual machine technology today is that organizations typically assume that this separation of guest and host systems is impenetrable: “This vulnerability provides an important wake-up call that virtualization is not immune to security flaws and that ‘real’ environments aren’t safe simply because they sit behind virtual environments."

“There seems to be a tendency to think that because virtualization provides isolation from the real operating system then you’re going to have a lot more security just by virtue of being virtualized,” he says. “But whenever you add software to a system and make it more complex, there are going to be more bugs.”

Although there are no known exploits in the wild, Arce says, Core found the bug while studying a previously disclosed (and since patched) related vulnerability, so it wouldn’t be tough for an attacker to find and use. “There’s a vulnerability similar to this one that’s been in exploitation for over a year now... so this one would not be hard to figure out” as well. The flaw is simple to exploit, he notes.

In a nutshell, the VMware software doesn’t properly “sanitize” malicious input into the file PathName parameter, according to Core, which reported the vulnerability to VMware in October.

VMware’s Chen argues that the vulnerability isn’t in the core virtualization technology itself. “The virtualization feature is not where the vuln exists. If a user wasn’t using Shared Folders, there would still be an isolation between [the guest and host OS]... and you can’t penetrate it [and] it’s not that likely to be [exploited].”

And when a user enables Shared Folders, he gets a warning message stating that using Shared Folders could expose the host to vulnerabilities, so there should be no surprises here, according to Chen. “They are already aware there’s a vulnerability between the guest OS and host OS” there.

Meanwhile, Core Security also has released an exploit for the vulnerability for use in its Core Impact penetration testing tool.

Workarounds for the bug include disabling Shared Folders altogether, or configuring it to read-only access to the host folder.

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