BookStack/app/Http/Middleware/CheckGuard.php

<?php

namespace BookStack\Http\Middleware;

use Closure;

class CheckGuard
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  \Closure  $next
     * @param string $allowedGuards
     * @return mixed
     */
    public function handle($request, Closure $next, ...$allowedGuards)
    {
        $activeGuard = config('auth.method');
        if (!in_array($activeGuard, $allowedGuards)) {
            session()->flash('error', trans('errors.permission'));
            return redirect('/');
        }

        return $next($request);
    }
}