If you using a plugin to implement a CAPTCHA on the WordPress login form (the backend login, wp-login.php), you will find that this is incompatible with the WP-Members front end login.

What happens is that in order to implement the CAPTCHA, the plugin must remove the function wp_authenticate_username_password from the authenticate process with remove_filter.  This allows the plugin to implement its own authentication that includes not only username and password, but also will validate the CAPTCHA.

This creates a problem for WP-Members which also uses WP’s authentication to log a user in because the WP-Members form will only be passing two parameters, username and password, and the authentication is now requiring the addition of a third parameter (the CAPTCHA).

Obviously, one solution is to not use the CAPTCHA plugin in the first place (or if it is a plugin that does more, disable the login CAPTCHA).  CAPTCHAs were never intended to be used for logins anyway and in my opinion it makes for a bad user experience while providing a very limited amount of “security.”  But I understand that is an opinion and there is room for other opinions of a differing view.  So if you must have a CAPTCHA on the login, then the solution for this is to turn the original WordPress process back on using add_filter to apply the original WP authenticate function; a process I will describe below.

Most CAPTCHA plugins apply their filter at a priority later than the default (10).  If we set our add_filter later on down the line (99 in this example), we will add the filter back after the CAPTCHA plugin has removed it (if you add it back before the CAPTCHA plugin removes it, this wouldn’t work).

add_filter( 'authenticate', 'wp_authenticate_username_password', 99, 3 );

But that alone isn’t really enough.  Just adding the filter back in is the equivalent of turning off the CAPTCHA plugin for the wp-login.php form.  So what’s the point of that, since the plugin has a setting to turn that process off anyway?

We still want the CAPTCHA to function on wp-login.php but not interfere with the front side WP-Members login.  To do that, we can add a condition to the process.  If we check to see if “wp-submit” is not set, we would know that the login is not coming from wp-login.php, and thus we would want to add the filter back in:

if( ! isset( $_POST['wp-submit'] ) ) {
	add_filter('authenticate','wp_authenticate_username_password',99,3);
}

Like most other filters, you can add this code snippet to your theme’s functions.php file or a custom functions plugin file.  There is information on this site on how to use code snippets.

Using this method, you can have a CAPTCHA on your wp-login.php form and still have WP-Members login front side login forms remain functional.