Security organizations are tracking what's being described as the largest email attack since last year's Warezov outbreak, and the second onslaught this week to steal a page from the Storm Trojan's playbook.

Adam Swidler, senior manager of solutions marketing for San Carlos, Calif.-based security vendor Postini Inc., said bot herders are using the outbreak to expand their array of zombie machines.

Those machines can then be used to push out spam, steal sensitive data from infected computers or launch other types of attacks. Initial reports from Postini's global data centers indicate that Thursday's outbreak has driven malware levels 60 times higher than average daily levels on the Internet, he added.

"IT shops need to block executable and .zip files, and users should never open an attachment from someone they don't know and trust," he said.

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