Source

File: wp-includes/pluggable.php

function wp_validate_auth_cookie( $cookie = '', $scheme = '' ) {
		$cookie_elements = wp_parse_auth_cookie( $cookie, $scheme );
		if ( ! $cookie_elements ) {
			/**
			 * Fires if an authentication cookie is malformed.
			 *
			 * @since 2.7.0
			 *
			 * @param string $cookie Malformed auth cookie.
			 * @param string $scheme Authentication scheme. Values include 'auth', 'secure_auth',
			 *                       or 'logged_in'.
			 */
			do_action( 'auth_cookie_malformed', $cookie, $scheme );
			return false;
		}

		$scheme     = $cookie_elements['scheme'];
		$username   = $cookie_elements['username'];
		$hmac       = $cookie_elements['hmac'];
		$token      = $cookie_elements['token'];
		$expired    = $cookie_elements['expiration'];
		$expiration = $cookie_elements['expiration'];

		// Allow a grace period for POST and Ajax requests.
		if ( wp_doing_ajax() || 'POST' === $_SERVER['REQUEST_METHOD'] ) {
			$expired += HOUR_IN_SECONDS;
		}

		// Quick check to see if an honest cookie has expired.
		if ( $expired < time() ) {
			/**
			 * Fires once an authentication cookie has expired.
			 *
			 * @since 2.7.0
			 *
			 * @param string[] $cookie_elements An array of data for the authentication cookie.
			 */
			do_action( 'auth_cookie_expired', $cookie_elements );
			return false;
		}

		$user = get_user_by( 'login', $username );
		if ( ! $user ) {
			/**
			 * Fires if a bad username is entered in the user authentication process.
			 *
			 * @since 2.7.0
			 *
			 * @param string[] $cookie_elements An array of data for the authentication cookie.
			 */
			do_action( 'auth_cookie_bad_username', $cookie_elements );
			return false;
		}

		$pass_frag = substr( $user->user_pass, 8, 4 );

		$key = wp_hash( $username . '|' . $pass_frag . '|' . $expiration . '|' . $token, $scheme );

		// If ext/hash is not present, compat.php's hash_hmac() does not support sha256.
		$algo = function_exists( 'hash' ) ? 'sha256' : 'sha1';
		$hash = hash_hmac( $algo, $username . '|' . $expiration . '|' . $token, $key );

		if ( ! hash_equals( $hash, $hmac ) ) {
			/**
			 * Fires if a bad authentication cookie hash is encountered.
			 *
			 * @since 2.7.0
			 *
			 * @param string[] $cookie_elements An array of data for the authentication cookie.
			 */
			do_action( 'auth_cookie_bad_hash', $cookie_elements );
			return false;
		}

		$manager = WP_Session_Tokens::get_instance( $user->ID );
		if ( ! $manager->verify( $token ) ) {
			/**
			 * Fires if a bad session token is encountered.
			 *
			 * @since 4.0.0
			 *
			 * @param string[] $cookie_elements An array of data for the authentication cookie.
			 */
			do_action( 'auth_cookie_bad_session_token', $cookie_elements );
			return false;
		}

		// Ajax/POST grace period set above.
		if ( $expiration < time() ) {
			$GLOBALS['login_grace_period'] = 1;
		}

		/**
		 * Fires once an authentication cookie has been validated.
		 *
		 * @since 2.7.0
		 *
		 * @param string[] $cookie_elements An array of data for the authentication cookie.
		 * @param WP_User  $user            User object.
		 */
		do_action( 'auth_cookie_valid', $cookie_elements, $user );

		return $user->ID;
	}