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The cybercrooks attempting to defeat CAPTCHAs are no longer just traditional junk-mailers who want to get around the test to send spam. In a recent study, security researchers have discovered that criminals are also using circumvention techniques in attacks that harvest financial or personal data.

A CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is commonly used to distinguish human users from computer automated applications, thus helping to prevent automated tools from abusing online services, such as webmail accounts. Hackers have developed numerous methods to bypass CAPTCHAs, including computer-assisted tools and crowd-sourcing, creating a cat-and-mouse game between miscreants and CAPTCHA providers such as Google and others.

Junk mailers, for example, are interested in defeating CAPTCHA challenges in order to establish webmail accounts for subsequent spam runs. Last weekend spammers managed to spam the UK's open data website by circumventing its CAPTCHA gateway in a slightly more sophisticated variant of the same play.

How do they do it?

Hackers are using computer-assisted tools based on optical character recognition or machine learning technologies as well as tools which outsource CAPTCHA-breaking to modern day sweatshops, typically located in India. More recently miscreants have begun hoodwinking naive users into being a part of the crowd sourced for CAPTCHA solutions. These crowd-sourcing techniques sometimes pose as CAPTCHA-busting games that reward players. Some CAPTCHA-busting sites offer free porn as an incentive.

Not just about spam anymore

However hackers might also be interested in circumventing CAPTCHAs as a means to collect financial or personal details, according to the new study by data security firm Imperva.

Attacks based on CAPTCHA-busting have now been used to access a system for filing financial status reports maintained by one of the central banks in Argentina. Criminals have also launched attacks designed to obtain tax details associated with a Brazilian social security number. Hackers have also targeted the website of an agency in charge of the voting process in Brazil. All three sets of attacks are likely one important part in a more elaborate set of scams, most likely involving ID theft.

In response, CAPTCHA providers need to step up their game to make life harder for miscreants. Approaches on offer include delivering more difficult CAPTCHAs to potentially suspicious users or integrating simple riddles and contextual semantics to beef up challenges. Approaches such as traffic-based automation detection, behavioural analysis, content analysis and blacklists can help distinguish suspicious parties from genuine surfers.

Improvements to the security of CAPTCHA can be made without making life too difficult for legitimate users, according to Imperva.

"CAPTCHA security, like many other security segments, is a battle of innovation between hackers and security professionals," said Amichai Shulman, CTO of Imperva. "CAPTCHA security must be balanced against a positive user experience, but can readily be improved by deploying anti-automation solutions to help prevent hackers from employing anti-CAPTCHA tools."

The June edition of Imperva's Hacker Intelligence report, A CAPTCHA in the Rye explaining the threat in greater depth, can be found here. ®

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