During a recent email conversation with several participants, we discovered that the email service of one participant silently dropped legitimate emails that happened to contain certain combinations of words common in spam. I believe this sort of filter is common practice, and in fact even in place for some of my own email addresses.

However, this experience made me think: isn't predictable spam filtering in general a vulnerability that could be used as a hoax device? Since most users reply to an email citing the complete source email, including filter-offending words, it should be possible to keep a reply, forward, or even a whole thread, under the radar of specific recipients. If used in combination with forged replies from addresses predictably dropping emails, I think this may be a dangerous tool for social engineering.

For example: attacker 'A' sends 'B' a social engineering request for "the secret plans" and says "if you are unsure, forward my request to your boss and ask if this is okay". 'B' forwards the email to his boss 'C' and asks "Is this okay?". However, 'C':s spam filter silently drops the email. 'A' forges a reply from 'C' saying: "Sure, no problem, go ahead."

Regards,
R. Armiento

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