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.
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:
- If the target site sets
SameSite=None on its auth cookie and the user visits a malicious site in a Chromium-based browser
- 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.
- 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.
References
Summary
The SvelteKit framework offers developers an option to create simple REST APIs. This is done by defining a
+server.jsfile, 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-casedContent-Typeheader 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_requestsDetails
The CSRF protection is implemented using the code shown below.
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-Typeheader whether the value is eitherapplication/x-www-form-urlencoded,multipart/form-dataortext/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 theContent-Typeheader, is not sufficient to mitigate all possible variations of this type of attack. Since this function is checkingContent-Typewith lower-cased values, and the browser accepts upper-casedContent-Typeheader to be sent, a CSRF attack performed with theContent-Typeheader that contains an upper-cased character (e.g.,text/plaiN) can circumvent the protection 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. This may lead to all POST operations requiring authentication being allowed in the following cases:
SameSite=Noneon its auth cookie and the user visits a malicious site in a Chromium-based browserSameSiteattribute explicitly and the user visits a malicious site with Firefox/Safari with tracking protections turned off.Remediations
It is preferred to update to SvelteKit 1.15.2. It is also recommended to explicitly set
SameSiteto a value other thanNoneon authentication cookies especially if the upgrade cannot be done in a timely manner.References