Default Response Headers

Since Thelia 3.1, every response to a main request carries three security headers:

Header	Value	What it does
X-Content-Type-Options	nosniff	The browser trusts the declared content type instead of guessing it.
X-Frame-Options	SAMEORIGIN	Another site cannot put the shop in a frame.
Referrer-Policy	strict-origin-when-cross-origin	A link out carries the origin, not the full path and query string.

They are set only when absent, so a theme, a module or a reverse proxy that writes its own value keeps the last word. Thelia\Core\EventListener\ResponseHeadersListener does the work, on the kernel response event, at priority -1024 so that everything else has already had its say.

Sub-requests are left alone: only the main request gets the headers.

When you have to override one

A shop that is displayed inside a frame on another domain has to write its own X-Frame-Options, or the browser will refuse to render it. Set the header before the listener runs, from a listener of your own on the same event at any priority above -1024:

use Symfony\Component\EventDispatcher\Attribute\AsEventListener;
use Symfony\Component\HttpKernel\Event\ResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;

#[AsEventListener(event: KernelEvents::RESPONSE)]
final readonly class PartnerFrameHeaders
{
    public function __invoke(ResponseEvent $event): void
    {
        if (!$event->isMainRequest()) {
            return;
        }

        $headers = $event->getResponse()->headers;
        $headers->set('Content-Security-Policy', "frame-ancestors 'self' https://partner.example.com");
        $headers->set('X-Frame-Options', 'SAMEORIGIN');
    }
}

The listener sees X-Frame-Options already set and leaves it alone, and the browser reads the frame-ancestors directive, which wins over X-Frame-Options wherever both are understood. That is the only portable way to name another host: X-Frame-Options itself knows nothing beyond SAMEORIGIN and DENY.

Setting the header at the reverse proxy works just as well: the listener does not overwrite a header the response already carries, and a proxy that adds its own after PHP has answered wins by definition.