@sveltejs/kit: SvelteKit framework has Insufficient CSRF protection for CORS requests

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 its users. The protection is implemented at kit/src/runtime/server/respond.js. While the implementation does a sufficient job of mitigating common CSRF attacks, the protection can be bypassed by simply specifying an upper-cased Content-Type header value. The browser will not send uppercase characters on form submission, but this check does not block all expected cross-site requests: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#simple_requests

Details

The CSRF protection is implemented using the code shown below.

		const forbidden =
			is_form_content_type(request) &&
			(request.method === 'POST' ||
				request.method === 'PUT' ||
				request.method === 'PATCH' ||
				request.method === 'DELETE') &&
			request.headers.get('origin') !== url.origin;

		if (forbidden) {
			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/PUT/PATCH/DELETE method, the protection will compare the server’s origin with the value of the HTTP Origin header. 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 whether the value is either application/x-www-form-urlencoded, multipart/form-data or text/plain. If all the previous checks pass, the request will be rejected with an 403 error response. However, is_form_content_type, which is responsible for checking the value of the Content-Type header, is not sufficient to mitigate all possible variations of this type of attack. Since this function is checking Content-Type with lower-cased values, and the browser accepts upper-cased Content-Type header to be sent, a CSRF attack performed with the Content-Type header that contains an upper-cased character (e.g., text/plaiN) can circumvent the protection and the request will be processed by the endpoint handler. <!--

PoC

1. Set up the SvelteKit with POST /api/test endpoint.
2. Replace REPLACE_DOMAIN in the following HTML.

<script>
(async() => {
	await fetch("https://REPLACE_DOMAIN/api/test", {method: "POST", headers:{"Content-Type":"text/plaiN"}, body: "hello=world"});
})();
</script>

1. Confirm that a POST request is processed by the server.

-->

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. This may lead to all POST operations requiring authentication being allowed in the following cases:

1. If the target site sets SameSite=None on its auth cookie and the user visits a malicious site in a Chromium-based browser
2. If the target site doesn't set the SameSite attribute explicitly and the user visits a malicious site with Firefox/Safari with tracking protections turned off.
3. If the user is visiting a malicious site with a very outdated browser.

Remediations

It is preferred to update to SvelteKit 1.15.2. It is also recommended to explicitly set SameSite to a value other than None on authentication cookies especially if the upgrade cannot be done in a timely manner.