A team of computer scientists at Stanford and Tulane University with expertise in artificial intelligence, audio processing, and computer security has come up with a way to automatically defeat the systems that prevent spammers from creating new accounts on sites like Yahoo, Microsoft's Hotmail, and Twitter.

Many websites require users to correctly transcribe a string of distorted characters—a puzzle known as a CAPTCHA—to gain access. These tests are relatively easy for people, but very hard for computers. Most sites also make CAPTCHAs available in audio form, for vision-impaired users, and the researchers found that their algorithm could solve many of these audio CAPTCHAs. Researchers at Carnegie Mellon University have demonstrated the vulnerability of audio CAPTCHAs before, in 2008, but the new work targets newer, more secure versions.

The ability to automatically defeat CAPTCHAs could make it cheaper for spammers to churn out spam. Right now, spammers pay humans sweatshop wages to solve CAPTCHAs, but this can cost up to one cent apiece.

Team leader Elie Bursztein, of Stanford University, says the team's algorithm, called deCAPTCHA, was able to defeat audio CAPTCHAs from Microsoft and Yahoo in almost half of all cases. Microsoft has since switched to another type of CAPTCHA, which the algorithm is still able to defeat in 1.5 percent of cases.