Sobig

Although there were indications that tests of the worm were carried out as early as August 2002, Sobig.A was first found in the wild in January 2003. Sobig.B was released on May 2003. It was first called Palyh, but was later renamed to Sobig.B after anti-virus experts discovered it was a new generation of Sobig. Sobig.C was released May 31 and fixed the timing bug in Sobig.B. Sobig.D came a couple of weeks later followed by Sobig.E in June 25. On August 19, Sobig.F became known and set a record in sheer volume of e-mails.

The worm was most widespread in its "Sobig.F" variant.

Sobig is a computer worm in the sense that it replicates by itself, but also a Trojan horse in that it masquerades as something other than malware. The Sobig worm will appear as an electronic mail with one of the following subjects:

Re: Approved
Re: Details
Re: Re: My details
Re: Thank you!
Re: That movie
Re: Wicked screensaver
Re: Your application
Thank you!
Your details
It will contain the text: "See the attached file for details" or "Please see the attached file for details." It also contains an attachment by one of the following names:

application.pif
details.pif
document_9446.pif
document_all.pif
movie0045.pif
thank_you.pif your_details.pif
your_document.pif
wicked_scr.scr
Technical detailsThe Sobig viruses infect a host computer by way of the above mentioned attachment. When this is started they will replicate by using their own SMTP agent engine. E-mail addresses that will be targeted by the virus are gathered from files on the host computer. The file extensions that will be searched for e-mail addresses are:

.dbx
.eml
.hlp
.htm
.html
.mht
.wab
.txt

Source : www.wikipedia.com