fix(fns): reject cross-site server function requests - #2321
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Server functions are same-origin RPC, but the handler ran them for any request to /_server regardless of origin or method, so another site could invoke a state-changing function with the visitor's cookies over a GET or a form POST. SameSite=Lax does not stop the GET vector, and the id can be given in the URL, so the custom-header preflight did not gate it either. Reject cross-site requests before the function runs: trust Sec-Fetch-Site when present, otherwise compare Origin against the request host. Same-origin, same-site, user-initiated navigations, and no-JS form posts keep working. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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brenelz
approved these changes
Sep 18, 2026
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Server functions are same-origin RPC, but the handler ran them for any request to /_server regardless of origin or method, so another site could invoke a state-changing function with the visitor's cookies over a GET or a form POST. SameSite=Lax does not stop the GET vector, and the id can be given in the URL, so the custom-header preflight did not gate it either.
Reject cross-site requests before the function runs: trust Sec-Fetch-Site when present, otherwise compare Origin against the request host. Same-origin, same-site, user-initiated navigations, and no-JS form posts keep working.