Strengthening CAPTCHA-based Web security


  • Graeme Baxter Bell Applied Artificial Intelligence Group at Murdoch University (external researcher)



Web security, CAPTCHA, Abuse of websites and services, CAPTCHA Improvement Ratio


Simple, universally applicable strategies can help any captcha–protected system resist automated attacks and can improve the ability of administrators to detect attacks. The strategies discussed here cause an exponential increase in the difficulty faced by automated attackers, while only increasing the inconvenience for human users in an approximately linear manner. These strategies are characterised using a new metric, the ‘Captcha Improvement Ratio’. The paper concludes that presenting multiple captcha systems together in random order may provide quantitative and qualitative advantages over many typical present–day captcha systems.

Author Biography

Graeme Baxter Bell, Applied Artificial Intelligence Group at Murdoch University (external researcher)

Graeme Bell holds a Ph. D. in Computer Science from the University of St Andrews, UK. He was the top Science graduate from the University of St Andrews in 2001 and also the winner of the 2001 Scottish Young Software Engineer of the Year award. His research interests include artificial intelligence, bioinformatics, robotics, image-processing, steganography, and the Internet.




How to Cite

Bell, G. B. (2012). Strengthening CAPTCHA-based Web security. First Monday, 17(2).