The Sobig-C mass-mailing worm spreads via email - forwarding itself to email addresses found on the infected computer's hard drive - and network shares.
When forwarding itself on to other computer users the worm spoofs the "From:" field either using addresses found on the computer's hard drive or "email@example.com".
Infected emails contain subject lines such as "Re: Movie", "Re: Approved", or "Re: Your application" and contain the message "Please see the attached file". The worm can use a number of different attachment names including screensaver.scr, movie.pif and documents.pif.
"Many users are cautious of EXE and VBS files which arrive in their inbox, but may not realise that PIF and SCR files are equally capable of containing viruses," said Graham Cluley, senior technology consultant for Sophos Anti-Virus. "Businesses should automatically block all executable code at the email gateway - it's the easiest way to avoid infection by a brand new email-aware worm."
Sophos recommends companies consider blocking all executable code at their email gateway. It is rarely necessary to allow users to receive programs via email from the outside world. There is so little to lose, and so much to gain, simply by blocking all emailed programs, regardless of whether they contain viruses or not.
Further information about W32/Sobig-C and how to protect against it can be found at: http://www.sophos.com/virusinfo/analyses/w32sobigc.html