Computer virus writers attacked thousands of computers on Friday using an unusually topical e-mail citing raging European storms, a security company said.
The virus, which the company named "Storm Worm," was emailed to hundreds of thousands of addresses globally with the subject line "230 dead as storm batters Europe."
An attached file contained so-called malware that can infiltrate computer systems.
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Hypponen said thousands of computers, most in private use, had been affected.
He said most users would not notice the malware, or trojan, which creates a back door to the computer that can be exploited later to steal data or to use the computer to post spam.