Computer viruses are a common problem for computer users. One typical mode of attack is to send an electronic mail message (e-mail) containing a file attachment to an unsuspecting user's computer. The file attachment contains malicious attack code, and the e-mail may contain some inducement for the user to launch the file attachment. When the user clicks on the file attachment, the attack code embedded in the file is executed. The attack code accesses an address book and sends the file attachment in an e-mail to addresses found in the address book. The attack code may then try to modify files on the user's computer or obtain other files and mail them back to the attackers.
Propagation of such an attack is rapid. Once one unsuspecting user launches the file attachment, the virus quickly spreads to other unsuspecting users, who then perpetuate the problem. Such viruses have been known to overwhelm computer networks and cause millions of dollars of damage to network operators, companies, and users.
Techniques exist to purge viruses from affected computers. Such techniques, however, are often are used only after the virus has been detected and many computers have been infected. New methods are desired that will slow down the propagation of computer viruses and other malicious code, thus allowing the virus detectors to detect and delete the viruses before the damage becomes widespread.
Along with improving the ability to detect and slow such attacks, it is also desirable to prevent or limit damage to users' systems and access to users' data. The ideal world in which users would never run suspicious files will never exist, so a practical solution must recognize this and attempt to prevent or limit these programs from damaging the user's system and accessing the user's data.