The U.S. Department of Defense was hit with a severe malware attack this week. The attack, which originated in Russia, was targeted at the networks in the department’s Central Command, which oversees the U.S.’s involvement in Iraq and Afghanistan. DOD sources say at least one highly classified network was compromised. According to the Los Angeles Times, the malware has been around awhile:

          The invasive software, known as agent.btz, has circulated among non-governmental U.S. computers for months. But only recently has it affected the Pentagon’s networks. It is not clear whether the version responsible for the cyber-intrusion of classified networks is the same as the one affecting other computer systems.

The malware is able to spread to any flash drive plugged into an infected computer. The risk of spreading the malware to other networks prompted the military to ban the flash drives.

Agent.btz spreads via removable media and once installed it contacts a remote server and downloads additional malware. DOD officials declined to give further details, citing security concerns, but say they have not yet determined whether an organization or a single hacker is responsible for the attack.

The fact that a hacker or group of hackers was able to easily penetrate the Department of Defense’s network has raised serious concerns about its computer security practices. It’s not yet know what, if any, data was stolen as a result of the attack.