SvelteKit (@sveltejs/kit) from 2.10.0 before 3.0.0 contains a path traversal vulnerability in the vite preview server on Windows that allows unauthenticated attackers to read files outside the prerendered output directory. Attackers who can reach the preview server through network access or a tunnel can send crafted requests to expose source code, configuration, or credentials readable by the server process.

Impact

Applications that prerender pages and use setHeaders to set a cache-control header from an untrusted source are vulnerable to cross-site scripting (XSS).

SvelteKit inserts the header value into a <meta http-equiv="cache-control"> tag without escaping it. An attacker who can influence this value at prerender time can break out of the content attribute and inject HTML or JavaScript into the generated page. The injected code can then execute when users visit the page.

Patches

The fix escapes the header value before inserting it into the generated HTML. Upgrade to @sveltejs/kit version 3.0.0 or later.

Workarounds

Avoid forwarding untrusted cache-control header values with setHeaders on prerendered pages. Use a fixed, trusted cache policy instead.

Impact Applications using enhanced form actions (use:enhance) on routes that accept paths beginning with //, such as catch-all routes, may be vulnerable to an open redirect.

Patches A fix has been prepared for @sveltejs/kit 3.0.0. Users should upgrade to 3.0.0 or later.

The fix preserves leading double slashes in action paths without generating protocol-relative URLs, restricts non-redirect action navigation to the current origin, and includes the origin when comparing action locations with the current page.

Workarounds Reject requests whose parsed URL pathname begins with // before route resolution, for example in a reverse proxy or a SvelteKit handle hook.

Alternatively, disable navigation for non-redirect enhanced form results by supplying a custom submission callback:

svelte <form method="POST" use:enhance={() => ({ update }) => update({ navigate: false })}> <!-- form fields --> </form>

This applies non-redirect results to the current page instead of navigating to the action location.

SvelteKit (@sveltejs/kit) versions >=2.49.0 and <=2.52.1 with experimental remote functions and form enabled contain a CPU exhaustion vulnerability in form deserialization. An attacker can send malformed form data to cause the server to become unresponsive while processing the request, resulting in denial of service. Fixed in 2.52.2.

SvelteKit (@sveltejs/kit) versions >=2.49.0 and <=2.52.1 with experimental remote functions (experimental.remoteFunctions) and form enabled contain a memory exhaustion vulnerability in remote form deserialization. Malformed form data can cause excessive memory allocation, crashing the server process and resulting in denial of service. Fixed in 2.52.2.