Spammers adopt slippery tactics to bypass ISP defences

Botnets repurposed

By John Leyden • Get more from this author

Posted in Spam, 4th March 2005 16:32 GMT

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Spam levels are rising even though the percentage of junk mail spewed out from compromised PCs directly is on the slide. Tests by email security firm MessageLabs on 90,000 inbound connections to its honeypot servers on 1 October 2004 revealed that 79 per cent of the connections came from "open proxy" computers or zombies (computers typically compromised by a virus or Trojan infection). The same tests on 1 February 2005 showed this number had dropped to 59 per cent.

Messagelabs stats showed spam levels had risen from 72 per cent of inbound email traffic in September 04 to 83 per cent of inbound email by January 2005. Put together the findings provide evidence that spammers are changing tactics in order to get around the defences established by ISPs.

"Some ISPs now routinely block TCP port 25 (SMTP) traffic, or even force email traffic to be sent via the ISPs own email servers, which in turn is forcing the spammers to become more innovative with their techniques," said Paul Wood, chief information security analyst at MessageLabs. "Software such as Send-Safe with its recent "proxy lock" feature now enables the spammers to instruct the bots to relay the spam through each bot's ISP email server, rather than to attempt delivery itself, and the benefits to the spammer can be clearly seen in these results." ®