I'm trying to create security question field for my login page

Question

For the last 2 hours I've trying to create a mu plugin that requires the user to answer a question first, so he can login in the website.

It is "What is the name of the white horse of Napoleon?", so the user replies "white" and it should work, by theory. The problem is that it seems it is not getting validated during the login. This is my code:

 <?php

add_action( 'login_form', 'add_login_field' );
function add_login_field() { ?>
    <p>
        <label><?php _e('What is the name of the white horse of Napoleon?') ?><br />
        <input type="text" name="user_proof" id="user_proof" class="input" size="25" tabindex="20" /></label>
    </p>
<?php }

add_action( 'login_post', 'add_login_field_validate', 10, 3 );

function add_login_field_validate( $sanitized_user_login, $user_email, $errors) {
    if (!isset($_POST[ 'user_proof' ]) || empty($_POST[ 'user_proof' ])) {
        return $errors->add( 'proofempty', '<strong>ERROR</strong>: You did not answer the proof-of-humanship question.'  );
    } elseif ( strtolower( $_POST[ 'user_proof' ] ) != 'white' ) {
        return $errors->add( 'prooffail', '<strong>ERROR</strong>: You did not answer the proof-of-humanship question correctly.'  );
    }
}

?>

I believe the problem is with the part:

add_action( 'login_post', 'add_login_field_validate', 10, 3 );

Can anyone give me a hand here?

Answer 1

There is no login_post action that I know of - use the authenticate filter instead, inside wp_authenticate(), called by wp_signon():

function wpse_185339_check_user_answer( $user ) {
    if ( $_SERVER['REQUEST_METHOD'] === 'POST' ) {
        if ( empty( $_POST[ 'user_proof' ] ) ) {
            $user = new WP_Error( 'proofempty', '<strong>ERROR</strong>: You did not answer the proof-of-humanship question.' );
        } elseif ( strtolower( $_POST[ 'user_proof' ] ) !== 'white' ) {
            $user = new WP_Error( 'prooffail', '<strong>ERROR</strong>: You did not answer the proof-of-humanship question correctly.' );
        }
    }

    return $user;
}

add_filter( 'authenticate', 'wpse_185339_check_user_answer', 100 );