The BBC has become the first organisation to admit falling victim to the latest variant of the 'ExploreZip' worm, which was only detected yesterday.
The infection occurred at about 2pm yesterday and BBC staff were alerted with an email headed 'Major Incident Warning 38'.
"We can confirm that we've been infected with the ExploreZip virus," said a BBC spokesperson. "Our IT staff are working round the clock to solve the problem, and we are confident we'll have it under control by the weekend."
Meanwhile, the BBC's IT support line carries a message warning staff that all emails have been limited to 70Kb, and all antivirus software has been updated.
Once 'ExploreZip.E' gets onto a PC it emails itself out as an attachment in a reply to all read and unread emails in Microsoft Outlook with the following message:
'Hi I have received your email and I shall send you a reply ASAP. Till then take a look at the attached zipped docs. bye.'
A file called ZIPPED_FILES.EXE is attached which contains the worm.
Once the mails are sent the worm overwrites all Microsoft Word, Excel and PowerPoint files on a PC and reduces their size to 0KB.
This makes recovery of the information without back-ups very difficult, more so than if the files were simply deleted.
"It is vital to keep your antivirus software updated regularly, even hourly, for the best security," said Carole Theriault, antivirus consultant at Sophos.
"If you do get caught this kind of virus demonstrates the importance, and payback, of a solid back-up policy."
All the major antivirus software vendors now have definitions and a fix available for ExploreZip.
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