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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 on 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 not 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:
The 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:
The Sobig.F variant was programmed to contact 20 IP addresses on UDP port 8998 on August 26, 2003 to install some program or update itself. It is unclear what this program was, but earlier versions of the virus had installed the WinGate proxy server software - a legitimate product - in a configuration allowing it to be used as a backdoor for spammers to distribute unsolicited e-mail.
The Sobig worm was written using the Microsoft Visual C++ compiler, and subsequently compressed using a data compression program called tElock.
The Sobig.F worm reactivated itself on March 10, 2013. On March 14 the same year, Microsoft announced that they will pay $250,000 for information leading to the arrest of the creator of the Sobig worm. To date, the perpetrator has not been caught.