Worm Sobig is an Internet worm, which spreads by e-mail in letters with infected attachments. Once such attachment is executed, the worm installs itself to the system and distributes itself to e-mail addresses found in files of several types. It also infects vulnerable computers with shared resources in a local network. The worm contains a backdoor, which can be used to update it or install some plugins. Although this worm can cause a high overload of mail servers, it is outdated and doesn’t spread now. However, its backdoor can still be active and may be used by attackers.