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Prepending is the adding of a term, expression, or phrase to the beginning or header of some other communication. Often prepending is used in order to further refine or establish the pretext of a social engineering attack, such as spam, hoaxes, and phishing. An attacker can precede the subject of an attack message with RE: or FW: (which indicates “in regard to and forwarded,” respectively) to make the receiver think the communication is the continuance of a previous conversation rather than the first contact of an attack. Other often-used prepending terms are EXTERNAL, PRIVATE, and INTERNAL.

Prepending attacks can also be used to fool filters, such as spam filters, antimalware, firewalls, and intrusion detection systems (IDSs). This could be accomplished with SAFE, FILTERED, AUTHORIZED, VERIFIED, CONFIRMED, or APPROVED, among others. It might even be possible to interject alternate email header values, such as “X-Spam-Category: LEGIT” or “X-Spam-Condition: SAFE,” which could fool spam and abuse filters.