Next participant of the project is reCaptcha. It is popular captcha which used at many web sites.
As said at recaptcha.net, this captcha has plugins for many engines, such as: WordPress, MediaWiki, phpBB, Movable Type, Drupal, Symfony, Typo3, NucleusCMS, vBulletin, Joomla. This is popular external captcha service and there are many thousands of sites which are in risk with this captcha.
This captcha is vulnerable for one interesting methods of bypassing (I called it captcha token bypass method). This Insufficient Anti-automation hole I found 31.08.2007.
In captcha token bypass method you need to bypass tokens only, without answering at any captcha images. So you need to use only captcha_token parameter (and not use recaptcha_response_field parameter at all). For bypassing you need use new captcha token for every post.
I found this hole at www.keng.ws. As I tested at some others sites which use reCaptcha, they were not vulnerable to this hole. So it’s just incorrect implementation of captcha. But there still possible a lot of others sites with such holes (which not correctly setup reCaptcha).
Guys not overdo with this Captcha bypass test. Not post too much at this site. This exploit for educational purposes only.
Moral: never implement captchas incorrectly.
This vulnerability concerns only reCaptcha plugin for Drupal.