Department of Defense computers have been hit with an old worm that the DoD won't discuss other than to say it is taking steps to mitigate its effects.

One report identified the virus as W32/SillyFCD-W, which spreads via thumb drives that move from machine to machine. Once the virus is on a machine, it may download other malware, according to Sophos. Protection against the worm has been available from Sophos since April 2007.

Perhaps in a related move, the department has issued a directive banning thumb drives from its networks, leading to speculation that the virus may have been traced to one, reports say. Thumb drives have been a source of security concern.

While a military spokesman has confirmed an outbreak, he would not discuss details as per defense policy, according to published reports.

Banning thumb drives is a relatively routine policy for organizations interested in security, reliability and data loss prevention, one analyst says. "That's not necessarily an out of the ordinary policy, says Mike Montecillo, a security analyst for Enterprise Management Associates.

One concern is that employees can attach at thumb drive and download sensitive data and then walk off with it, he says. Or the thumb drive could be used on any number of machines, exposing it to infection that it can then spread. "Who knows what crept onto the thumb drive while it was attached?" he says.

He says military security teams will likely go about removing the worm from infected machines, which could simply mean running antivirus scans. "It's not that profound an impact on the network," he says.

Earlier this year, workers in the state of Washington's Division of Child Support were provided with state-owned USB flash drives as part of a move to eliminate the use of unsanctioned thumb drives.

Copyright 2008 IDG. All Rights Reserved.