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Laundering e-mail spam through open-proxies or compromised PCs is a widely-used trick to conceal real spam sources and reduce spamming cost in the underground e-mail spam industry. Spammers have plagued the Internet by exploiting a large number of spam proxies. The facility of breaking spam laundering and deterring spamming activities close to their sources, which would greatly benefit not only e-mail users but also victim ISPs, is in great demand but still missing. This paper reveals one salient characteristic of proxy-based spamming activities, namely packet symmetry, by analyzing protocol semantics and timing causality. Based on the packet symmetry exhibited in spam laundering, the paper proposes a simple and effective technique, DBSpam, to online detect and break spam laundering activities inside a customer network.
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