Source

File: wp-includes/pluggable.php

function wp_authenticate( $username, $password ) {
		$username = sanitize_user( $username );
		$password = trim( $password );

		/**
		 * Filters whether a set of user login credentials are valid.
		 *
		 * A WP_User object is returned if the credentials authenticate a user.
		 * WP_Error or null otherwise.
		 *
		 * @since 2.8.0
		 * @since 4.5.0 `$username` now accepts an email address.
		 *
		 * @param null|WP_User|WP_Error $user     WP_User if the user is authenticated.
		 *                                        WP_Error or null otherwise.
		 * @param string                $username Username or email address.
		 * @param string                $password User password
		 */
		$user = apply_filters( 'authenticate', null, $username, $password );

		if ( null == $user ) {
			// TODO: What should the error message be? (Or would these even happen?)
			// Only needed if all authentication handlers fail to return anything.
			$user = new WP_Error( 'authentication_failed', __( '<strong>Error</strong>: Invalid username, email address or incorrect password.' ) );
		}

		$ignore_codes = array( 'empty_username', 'empty_password' );

		if ( is_wp_error( $user ) && ! in_array( $user->get_error_code(), $ignore_codes, true ) ) {
			$error = $user;

			/**
			 * Fires after a user login has failed.
			 *
			 * @since 2.5.0
			 * @since 4.5.0 The value of `$username` can now be an email address.
			 * @since 5.4.0 The `$error` parameter was added.
			 *
			 * @param string   $username Username or email address.
			 * @param WP_Error $error    A WP_Error object with the authentication failure details.
			 */
			do_action( 'wp_login_failed', $username, $error );
		}

		return $user;
	}