Source

File: wp-includes/pluggable.php

function auth_redirect() {
		$secure = ( is_ssl() || force_ssl_admin() );

		/**
		 * Filters whether to use a secure authentication redirect.
		 *
		 * @since 3.1.0
		 *
		 * @param bool $secure Whether to use a secure authentication redirect. Default false.
		 */
		$secure = apply_filters( 'secure_auth_redirect', $secure );

		// If https is required and request is http, redirect.
		if ( $secure && ! is_ssl() && false !== strpos( $_SERVER['REQUEST_URI'], 'wp-admin' ) ) {
			if ( 0 === strpos( $_SERVER['REQUEST_URI'], 'http' ) ) {
				wp_redirect( set_url_scheme( $_SERVER['REQUEST_URI'], 'https' ) );
				exit;
			} else {
				wp_redirect( 'https://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'] );
				exit;
			}
		}

		/**
		 * Filters the authentication redirect scheme.
		 *
		 * @since 2.9.0
		 *
		 * @param string $scheme Authentication redirect scheme. Default empty.
		 */
		$scheme = apply_filters( 'auth_redirect_scheme', '' );

		$user_id = wp_validate_auth_cookie( '', $scheme );
		if ( $user_id ) {
			/**
			 * Fires before the authentication redirect.
			 *
			 * @since 2.8.0
			 *
			 * @param int $user_id User ID.
			 */
			do_action( 'auth_redirect', $user_id );

			// If the user wants ssl but the session is not ssl, redirect.
			if ( ! $secure && get_user_option( 'use_ssl', $user_id ) && false !== strpos( $_SERVER['REQUEST_URI'], 'wp-admin' ) ) {
				if ( 0 === strpos( $_SERVER['REQUEST_URI'], 'http' ) ) {
					wp_redirect( set_url_scheme( $_SERVER['REQUEST_URI'], 'https' ) );
					exit;
				} else {
					wp_redirect( 'https://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'] );
					exit;
				}
			}

			return; // The cookie is good, so we're done.
		}

		// The cookie is no good, so force login.
		nocache_headers();

		$redirect = ( strpos( $_SERVER['REQUEST_URI'], '/options.php' ) && wp_get_referer() ) ? wp_get_referer() : set_url_scheme( 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'] );

		$login_url = wp_login_url( $redirect, true );

		wp_redirect( $login_url );
		exit;
	}