Hackers have built a computer virus that attacks Siemens AG's widely used industrial control systems, creating malicious software that analysts said can be used for espionage and sabotage.The German company said the malware is a Trojan worm dubbed Stuxnet that spreads via infected USB thumb drives, exploiting a yet-to-be-patched vulnerability in Microsoft Corp's Windows operating system."Just viewing the contents of the USB stick can activate the Trojan," said Siemens spokesman Alexander Machowetz. "Siemens recommends avoiding the use of a USB stick." Siemens first learned of the problem on July 14, he said.

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