by Gunter Ollmann, Stefan Frei, Ivo Silvestri

Analysis of e-mail non-delivery receipt handling by live Internet bound e-mail servers has revealed a common implementation fault that could form the basis of a new range of DoS attacks. Our research in the field of email delivery revealed that mail servers may respond to mail delivery failure with as many non-delivery reports as there are undeliverable Cc: and Bcc: addresses contained in the original e-mail. Non-delivery notification e-mails generated by these systems often include a full copy of the original e-mail sent in addition to any original file attachments. This behavior allows malicious users to leverage these mail server implementations as force multipliers and flood any target e-mail system or account.

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