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Captcha-busting Trojan warning

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Spammers have come up with a sleazy - but undoubtedly ingenious - way to defeat anti-spam security checks.

The Captcha Trojan disguises itself as a stripper game that offers voyeurs the chance to see images of a model getting undressed. In order to get "Melissa" to lose an item of clothing, the user must identify the letters or numbers found within a scrambled text image that forms the basis of a captcha (Completely Automated Public Turing test to tell Computers and Humans Apart). Providing users identify the letters correctly, Melissa shows a bit more skin.

Captcha challenge-response systems, which are used to prevent accounts being created until a user correctly identifies letters in an image, are designed to ensure requests are made by a human rather than an automated program. In the case of the Captcha Trojan, the captcha comes from a website hackers wish to sign up to rather than the "game" itself.

So by deciphering the text, voyeurs are unwittingly helping crooks get around checks designed to stop them establishing accounts. These newly-created accounts may later be used to send junk mail or other malign purposes.

Captchas have been used to defeat automatic sign-ups to email accounts by services including Yahoo! Mail and GMail for years. Increasingly hackers have had more success at defeating the approach. For example, the HotLan Trojan has created more than 500,000 spam email accounts with Hotmail, Yahoo! and GMail since its arrival back in July.

Once a webmail account is established, encrypted spam emails are sent from a website onto infected machines. The HotLan Trojan then decrypts these junk emails and sends them to (presumably valid) addresses taken from yet another website. Generally these accounts get shut down following complaints after a few days, but in the meantime spammers have a resource to abuse.

Nice 'n sleazy

The Captcha Trojan represents an evolution in techniques designed to thwart Captcha challenge-response systems. The captchas seen in early versions of the Captcha Trojan program come from the sign-up system used by Yahoo!, according to an analysis of the package by anti-virus firm Trend Micro. "Although various methods of OCR (Optical Character Recognition) are already used to circumvent the CAPTCHA, this social engineering technique is new in that it uses people to unsuspectingly aid a malicious user," writes Trend researcher Roderick Ordoñez.

The virtual stripper game-cum-security-check-defeater most often appears on already infected machines. Even so, its prevalence is low and the few security firms to detect it so far (Trend Micro and Panda) classify it as a low-risk pathogen.

A Yahoo! representative told the BBC that it recognised the need to adapt its capatcha technology to deal with evolving spammer tactics. "Yahoo is continuing to innovate in our defenses against this type of abuse," a statement from the firm said. "We have a number of mechanisms to help us detect and respond to abuse." ®

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