SvelteKit: URL redirection to untrusted site via enhanced form submission

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:

<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.