Steps to reproduce the issue

Just use recaptcha plugin with version 2.0 and error reporting on maximum. Or try to submit rcaptcha using AJAX.

Expected result

Successfully submitted form.

Actual result

I submit form using AJAX and due to this error I akways get the same response:

Notice: Undefined variable: challenge in /mysite/plugins/captcha/recaptcha/recaptcha.php on line 158

This message will also pop, when error reporting is set to maximum.

System information (as much as possible)

Joomla 3.4, custom form.

Additional comments

Fix is very simple: $challenge variable should be defined any possible way. For example, you may add this line for version 2.0:

$challenge = $input->get('g-recaptcha-response', '', 'string');