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The growing abuse of webmail services to send spam has led anti-spam services to throttle messages from Gmail and Yahoo!

Over recent months security firms have reported that the Windows Live CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) used by Hotmail, and the equivalent system at Gmail, have been broken by automated attacks.

Obtaining a working Gmail account has a number of advantages for spammers. As well as gaining access to Google's services in general, spammers receive an 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.

Anti-spam filtering services such as MessageLabs have responded by throttling or slowing down the connection. "We're seeing more spam coming from Gmail and Yahoo!. Where a service is widely abused its reputation goes down and it's held back in the queue. This happens automatically," explained MessageLabs security analyst Paul Wood.

The approach, one stage in a multi-stage scanning and filtering process, is designed to make life difficult for spammers using botnets to send spam through compromised webmail accounts.

"These traffic management controls are not designed to block messages, they are intended only to slow down their transit. For messages that are subsequently blocked there should be a reason given in the non-delivery report," Wood told El Reg.

An analysis of spam trends in February 2008 (the last available monthly figures) by MessageLabs revealed that 4.6 per cent of all spam originates from web mail-based services.

The proportion of spam from Gmail increased two-fold from 1.3 per cent in January to 2.6 per cent in February, most of which spamvertised skin-flick websites. Yahoo! Mail was the most abused web mail service, responsible for sending 88.7 per cent of all web mail-based spam.

It was first thought that automated tools were used by spammers to defeat security checks and establish webmail accounts that might later be abused to send junk. More organisations are coming around to the theory, first floated by Brad Taylor, a Google software engineer, that bots are signing-up for accounts before sending the puzzles to real people.

Cyber criminals are employing sweatshops in India for as little as $4 a day to defeat anti-spam security checks, according to a recent analysis by net security firm Trend Micro. It reckons miscreants prefer to hire cheap labour rather than using automated techniques to defeat CAPTCHAs - that are only effective 30-35 per cent of the time - or malware-based approaches.

Previous systems to solve the CAPTCHA security check included the creation of a virtual stripper programme that awarded dupes with the removal of an item of clothing when they typed text shown in an accompanying image, served up from a web service sign-up CAPTCHA. ®

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