Propagation Mechanisms

While malicious code propagating over SMTP decreased during this period, all other vectors experienced an increase. This is likely the result of an effort by attackers to diversify the way their threats proliferate. Good email scanning applications and increased user knowledge of mass-mailing threats have reduced the effectiveness of email as a propagation mechanism. As a result, some attackers are resorting to other propagation mechanisms or incorporating additional propagation mechanisms into a mass-mailing worm.

Source: Symantec Internet Security Threat Report, Vol. XI