"We invest a lot in protecting and monitoring our systems to counter threats and ensure security, which includes a comprehensive response to viruses, worms, and other malware we discover," the spokesman .

Although the keylogger appears to be harmless, some security experts found news of the intrusion alarming.

"This is bad in so many ways," Richard Stiennon, chief research analyst with IT-Harvest in Birmingham, Mich., told PCWorld. "It indicates that the military is using completely insecure operating systems and practices for the critical function of controlling drones."

"These are deadly weapons that must work as required and only when required," he continued. "To have their command and control corrupted by apparently common malware is inexcusable."

He maintained that the hard drives on the infected machines should be restored from a clean image. "A removal tool cannot be trusted to completely remove a virus," he asserted. "The fact that they attempted several times to remove this malware indicates the sorry state of protection within this critical military system."