The most common type of CAPTCHA puzzle (a series of distorted letters and numbers on a Web page) is increasingly being cracked by smarter AI software.

Jeff Yan, a researcher at the University of Newcastle, has revealed a program capable of completing the textual CAPTCHAs (Completely Automated Public Turing test to tell Computers and Humans Apart) used to protect Microsoft’s Hotmail, MSN, and Windows Live services with a success rate of 60 percent.

Philippe Golle of the Microsoft Palo Alto Research Center has developed a program called Asirra that can correctly pass an image-based CAPTCHA.