JavaScript execution in mail via XBL — Mozilla

Georgi Guninski demonstrated that even with JavaScript disabled in mail (the default) an attacker can still execute JavaScript when a mail message is viewed, replied to, or forwarded by putting the script in a remote XBL file loaded by the message. The executed script could be used to alter or change the appearance of the message, and can act as a “mail-tap” to spy on the contents added to a reply or forward. For example, the attacker could make a provocative offer to a rival business and then watch the internal debate as it was forwarded and replied to.