A hash buster is a program that generates a string of text for insertion in a spam message so that, to a spam filter, the e-mail appears to be a different message each time it is sent. The text might appear in the Subject line, its From line, or after the message body, and might either be coherent text or gibberish. The latter is sometimes arranged in word-like formations to be less easily detected.
The hashing process, used by some spam filters, represents each message as a single number (known as a hash) to simplify comparison. Each number is then compared to those of other messages to determine if it matches a list of known spam messages or enough other messages to determine bulk e-mail status. However, a hash buster is only effective for spam filters that rely solely on hash comparison, and most such programs combine a number of approaches.