Breaking Google's audio CAPTCHA

The problem, as discovered by Wintercore Labs and published at the start of March is that there are repeatable patterns evident in the audio file and by applying a set of complex but straight forward processes, a library can be built of the basic signal for each possible character that can appear in the CAPTCHA.

Wintercore point to other audio CAPTCHAs that could be easily reversed using this technique, including the one for Facebook. The wider impact of this work might take some time to appear, but it provides an interesting proof of breaking audio CAPTCHAs. At the least, it shows that both of Google's CAPTCHA tools have now been defeated by software and it should only be a matter of time until the same can be said for Microsoft and Yahoo!'s offerings. Even with an effectiveness of only 90%, any failed CAPTCHA can easily be reloaded for a second try.

Even though Wintercore have declined to release their tool, the increasing awareness of the capabilities described should see competing tools emerge in the near future.