Anti-virus firms are warning of the outbreak of yet another mass-mailing virus.

Sobig-C, which spreads by email or (less commonly) network shares, has been intercepted more than 15,000 times since its first appearance on Saturday by message filtering outfit MessageLabs. This virus has now reached 'high level' outbreak status, the company reports.

The emergence of Sobig-C comes days after its spammer-friendly (http://www.theregister.co.uk/content/56/30808.html) predecessor, Sobig-B (AKA Palyh), reached its "expiry date". The worm was designed to spread only until 31 May.

Sobig-C can be easily recognised since although the name of the subject line or infectious attachment varies the body of the text is always "Please see the attached file".

Open the attachment and you machine becomes infected.

On infection, Sobig-C will search through files on the infected machine looking for email addresses. It sends a copy of itself to the addresses it finds using its own SMTP (Simple Mail Transfer Protocol) Engine with a spoofed email address in the "From" section so that is appears to come from either bill@microsoft.com or a genuine address filleted from a compromised PC.

Propagation of the worm also involves copying itself via network shares to standard Windows system folders.

As usual the worm has no effect on either Mac or Linux machines.

Write ups of the varmint by F-Secure (http://www.f-secure.com/v-descs/sobig_c.shtml) and Symantec (http://securityresponse.symantec.com/avcenter/venc/data/w32.sobig.c@mm.html) provide more detailed information. ®

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