Summary
SvelteKit vulnerable to Cross-Site Request Forgery
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.
A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones. Typical impact: state-changing actions performed as the victim without their consent.
CVE-2023-29003 has a CVSS score of 8.8 (High). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.15.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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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.
Frequently Asked Questions
- What is CVE-2023-29003? CVE-2023-29003 is a high-severity cross-site request forgery (CSRF) vulnerability in @sveltejs/kit (npm), affecting versions < 1.15.1. It is fixed in 1.15.1. A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones.
- How severe is CVE-2023-29003? CVE-2023-29003 has a CVSS score of 8.8 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of @sveltejs/kit are affected by CVE-2023-29003? @sveltejs/kit (npm) versions < 1.15.1 is affected.
- Is there a fix for CVE-2023-29003? Yes. CVE-2023-29003 is fixed in 1.15.1. Upgrade to this version or later.
- Is CVE-2023-29003 exploitable, and should I be worried? Whether CVE-2023-29003 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2023-29003 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2023-29003? Upgrade
@sveltejs/kitto 1.15.1 or later.