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By | Bryan Betts 6th July 2007 12:52

Turing test challenges spam filters

Fuzzy PDF menace

Spammers have turned a widely-used anti-spam trick - fuzzy text that computers cannot recognise - to their own advantage, according to the head of an anti-spam software developer.

The distorted text images are arriving in PDF files touting German penny stocks, in yet another iteration of the pump-and-dump scam that's been around for a while now.

What's different from earlier image spam is not only that these are PDFs, which adds an extra layer of complexity to the task of filtering out spam, but the text inside is deliberately distorted to make it extra-hard for computers to recognise.

Neil Cook, European technology chief at anti-spam specialist Cloudmark, called it "a kind of Turing test for spam filters".

He added: "We've been seeing a lot of PDF stock spams for the last 10 days or so, and there was another spike last night. Images are particularly easy for humans to pick up, but particularly hard for computers.

"These ones are distorted too - it's the same technique that websites use to keep spammers off by making visitors type in distorted text during registration, and now the spammers are using it on us."

Cook claimed that Cloudmark's spam fingerprinting technology is holding off the unwanted PDFs for now, but said that new detection techniques could be needed if image spam continues to grow. ®

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