A security researcher at Google is recommending computer users make several configuration changes to protect themselves against a previously unknown vulnerability that allows untrusted users to take complete control of systems running most versions of Microsoft Windows.

The vulnerability resides in a feature known as the Virtual DOS Machine, which Microsoft introduced in 1993 with Windows NT, according to this writeup penned by Tavis Ormandy of Google. Using code written for the VDM, an unprivileged user can inject code of his choosing directly into the system's kernel, making it possible to make changes to highly sensitive parts of the operating system.

"You can in theory write to memory segments that are otherwise considered highly trusted and sensitive," said Tom Parker, a director in the security consulting services group at Securicon, a Washington, DC-based security practice. "So for example, malware could possibly use it to install a key logger."

The vulnerability exists in all 32-bit versions of Microsoft OSes released since 1993, and proof-of-concept code works on the XP, Server 2003, Vista, Server 2008, and 7 versions of Windows, Ormandy reported. Presumably, Windows 2000 is also susceptible. Immunity, a Miami-based company that makes auditing software for security professionals, has already added a module exploiting the vulnerability to its product called Canvas. The exploit has been tested on all versions of Windows except for 3.1.

Ormandy said the security hole can easily be closed by turning off the MSDOS and WOWEXEC subsystems. The changes generally don't interfere with most tasks since they disable rarely-used 16-bit applications. He said he informed Microsoft security employees of the vulnerability in June.

"Regrettably, no official patch is currently available," he wrote. "As an effective and easy to deploy workaround is available, I have concluded that it is in the best interest of users to go ahead with the publication of this document without an official patch."

Microsoft security officials - who are already working double-duty responding to a potent Internet Explorer bug used to attack Google - said they are looking in to Ormandy's advisory and are not aware of attacks that target the reported vulnerability. ®