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Spammers, fresh from the success of cracking the Windows Live captcha used by Hotmail, have broken the equivalent system at Gmail.

Internet security firm Websense reports that miscreants have created bots which are capable of signing up and creating random Gmail accounts for spamming purposes, defeating Captcha-based defences in the process. It reckons the same group of spammers are behind both attacks.

Captcha (Completely Automated Public Turing test to tell Computers and Humans Apart) 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. The technique has been used to defeat automatic sign-ups to email accounts by services including Yahoo! Mail and Gmail for years, and hackers are increasingly successful in 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 2007.

Websense reckons the latest Gmail Captcha hack is the most sophisticated it has seen to date. Unlike Live Mail Captcha breaking, which involved just one zombie host doing the entire job, the Gmail breaking process involves two compromised hosts. Each of the two compromised hosts applies a slightly different technique to analysing Captcha, as explained in a posting by Websense.

Even using the two techniques, only one in every five Captcha-breaking requests are successful. It's a fairly low percentage, but one that's still more than workable in the case of automated attacks.

It sounds like a lot of effort, but gaining a working Gmail account has a number of advantages for spammers. As well as gaining access to Google's services in general, spammers gain a address whose domain is highly unlikely to be blacklisted, helping them defeat one aspect of anti-spam defences. Gmail also has the benefit of being free to use.

A wide range of Captcha-breaking services are hosted on a domain located in the US, Websense reports. The page includes a support page and payment advice along with an internal test page. ®

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