Summary

The SvelteKit framework offers developers an option to create simple REST APIs. This is done by defining a +server.js file, containing endpoint handlers for different HTTP methods.

SvelteKit provides out-of-the-box cross-site request forgery (CSRF) protection to it’s users. The protection is implemented at kit/src/runtime/server/respond.js#L52. While the implementation does a sufficient job in mitigating common CSRF attacks, the protection can be bypassed by simply specifying a different Content-Type header value.

Details

The CSRF protection is implemented using the code shown below.

const forbidden =
  // (1)
  request.method === 'POST' &&
  // (2)
  request.headers.get('origin') !== url.origin &&
  // (3)
  is_form_content_type(request);

if (forbidden) {
  // (4)
  const csrf_error = error(403, `Cross-site ${request.method} form submissions are forbidden`);
  if (request.headers.get('accept') === 'application/json') {
    return json(csrf_error.body, { status: csrf_error.status });
  }
  return text(csrf_error.body.message, { status: csrf_error.status });
}

If the incoming request specifies a POST method (1), the protection will compare the server’s origin with the value of the HTTP Origin header (2). A mismatch between these values signals that a potential attack has been detected. The final check is performed on the request’s Content-Type header (3) whether the value is either application/x-www-form-urlencoded or multipart/form-data (kit/src/utils/http.js#L71). If all the previous checks pass, the request will be rejected with an 403 error response (4).

The is_form_content_type validation is not sufficient to mitigate all possible variations of this type of attack. If a CSRF attack is performed with the Content-Type header set to text/plain, the protection will be circumvented and the request will be processed by the endpoint handler.

Impact

If abused, this issue will allow malicious requests to be submitted from third-party domains, which can allow execution of operations within the context of the victim's session, and in extreme scenarios can lead to unauthorized access to users’ accounts.

Remediation

SvelteKit 1.15.1 updates the is_form_content_type function call in the CSRF protection logic to include text/plain.

As additional hardening of the CSRF protection mechanism against potential method overrides, SvelteKit 1.15.1 is now performing validation on PUT, PATCH and DELETE methods as well. This latter hardening is only needed to protect users who have put in some sort of ?_method= override feature themselves in their handle hook, so that the request that resolve sees could be PUT/PATCH/DELETE when the browser issues a POST request.