We report a novel attack on two CAPTCHAs that have been widely deployed on the Internet, one being Google's home design and the other acquired by Google (i.e. reCAPTCHA). With a minor change, our attack program also works well on the latest ReCAPTCHA version, which uses a new defence mechanism that was unknown to us when we designed our attack. This suggests that our attack works in a fundamental level. Our attack appears to be applicable to a whole family of text CAPTCHAs that build on top of the popular segmentation-resistant mechanism of "crowding character together" for security. Next, we propose a novel framework that guides the application of our well-tested security engineering methodology for evaluating CAPTCHA robustness, and we propose a new general principle for CAPTCHA design.

@techreport{elahmad2011robustness, author = {El Ahmad, Ahmad S and Yan, Jeff and Tayara, Mohamad}, title = {The Robustness of Google CAPTCHAs}, year = {2011}, url = {http://homepages.cs.ncl.ac.uk/jeff.yan/google.pdf}, keywords = {recognition, google, captcha, character, image, ocr, segmentation}, abstract = {We report a novel attack on two CAPTCHAs that have been widely deployed on the Internet, one being Google's home design and the other acquired by Google (i.e. reCAPTCHA). With a minor change, our attack program also works well on the latest ReCAPTCHA version, which uses a new defence mechanism that was unknown to us when we designed our attack. This suggests that our attack works in a fundamental level. Our attack appears to be applicable to a whole family of text CAPTCHAs that build on top of the popular segmentation-resistant mechanism of "crowding character together" for security. Next, we propose a novel framework that guides the application of our well-tested security engineering methodology for evaluating CAPTCHA robustness, and we propose a new general principle for CAPTCHA design. } }

%0 = techreport %A = El Ahmad, Ahmad S and Yan, Jeff and Tayara, Mohamad %B = } %C = %D = 2011 %I = %T = The Robustness of Google CAPTCHAs} %U = http://homepages.cs.ncl.ac.uk/jeff.yan/google.pdf