Systems used by the US State Department to check visa applications against criminal and terrorist records were rendered inoperable for nine hours yesterday because of a computer virus infection.

A virus hobbled the government's Consular Lookout and Support System (CLASS) which contains 15 million records from numerous US federal organisations, including a list of 78,000 suspected terrorists. Technicians had to shut down the unclassified network in order to cleanse systems of infection, AP reports.

Interviews with prospective visa applicants continued during the outage; however, checks on applications could not be processed, nor visas issued, until the system was restored online.

The exact cause of the infection remains unclear. However, AP reports that a message sent to US embassies and consular offices yesterday warned that the "Welchia" virus had been detected at one facility.

The Welchia worm spreads using the same flaw with Windows RPC subsystem infamously exploited by the Blaster worm. Welchia attempts to patch vulnerable systems. However its scanning behaviour can swamp out normal network traffic.

Collectively, viruses from the Blaster family have infected hundreds of thousands of computers worldwide. ®

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