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The robustness of CAPTCHAs

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No matter whether you like or hate it, CAPTCHA has found widespread application on numerous commercial web sites – it is now almost a standard security mechanism for defending against undesirable or malicious Internet bot programs.

This talk introduces our recent work on attacking numerous widely deployed CAPTCH As. I will present new techniques of general value to attack a number of text CAPTCH As, including the schemes designed and deployed by Microsoft, Yahoo and Google. In particular, the Microsoft CAPTCHA has been deployed since 2002 at many of their online services including Hotmail, MSN and Windows Live. Designed to be segmentation-resistant, this scheme has been studied and tuned by its designers over the years. However, our simple attack has achieved a segmentation success rate of higher than 90% against this scheme. It took on average ~80 ms for the attack to completely segment a challenge on an ordinary desktop computer. As a result, we estimate that this CAPTCHA could be instantly broken by a malicious bot with an overall (segmentation and then recognition) success rate of more than 60%. On the contrary, the design goal was that automated attacks should not achieve a success rate of higher than 0.01%. For the first time, our work shows that CAPTCH As that are carefully designed to be segmentation-resistant are vulnerable to novel but simple attacks.

Our experience suggests that CAPTCHA will go through the same process of evolutionary development as cryptography, digital watermarking and the like, with an iterative process in which successful attacks lead to the development of more robust systems.

This talk is part of the Computer Laboratory Security Group meeting presentations series.

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