SAN FRANCISCO -- A computer virus rampaged through corporate systems, devouring files, crippling e-mail systems and affecting thousands of computers Tuesday, according to anti-virus experts.
The Mini-Zip virus, related to one that caused a serious outbreak in June, was expected to renew its assault Wednesday morning as unsuspecting users checked their e-mail inboxes.
Sal Viveros, a marketing manager for Santa Clara-based Network Associates, which makes the McAfee anti-virus software, said some 20 large corporations had been affected by Tuesday evening.
Schrader said he fielded complaints of significant problems from four Fortune 500 companies and scores of smaller companies.
Mini-Zip is the worst outbreak since its parent bug, Worm.Explore.Zip, struck last summer. Worm.Explore.Zip was considered the most destructive virus since the infamous Melissa outbreak in the spring.
"The last time this virus came along it affected tens of thousands -- maybe hundreds of thousands of computers and caused millions of dollars in damage," said Dan Schrader, vice president of new technology at Trend Micro in Cupertino. "It's malicious and fast-spreading. We consider this to be high-risk."
Anti-virus experts said the bug gets loose from an infected system as a seemingly friendly reply to a clean e-mail sent via the Microsoft Outlook, Outlook Express or Exchange browsers.
The virus intercepts the original message and automatically sends itself as a response -- even changing the subject line from, for example, "Work Meeting" to "Re: Work Meeting."
The body of the message reads: "Hi (recipient's name)! I received your e-mail and I shall send you an e-mail ASAP. Till then, take a look the attached zipped docs. bye."
The e-mail contains an attachment called "zipped--files.exe." If a user double-clicks on the attachment, the virus is set loose in the new victim's system.
It then destroys a series of files in a computer's hard drive by replacing them with empty files.
Anti-virus experts cautioned users against opening e-mails if they do not know the sender or why they were sent. They said the virus could be fought with updated antiviral software.