cross-site post form submissions are forbidden astro

TL;DR

Astro's CSRF middleware (checkOrigin, on by default since Astro 4.9) returns a 403 with this exact message when it rejects a form POST. Most of the time the submission is legitimate and only looks cross-site because the app runs behind a reverse proxy: declare your production domain in security.allowedDomains in astro.config so Astro trusts the proxy's forwarded headers, rebuild, and the forms go through.

Cross-site POST form submissions are forbidden

Use this when

• Every form POST 403s in production with this exact message while the same forms work in local dev
• The app sits behind Cloudflare, Cloud Run, Caddy, or any TLS-terminating proxy
• Login, sign-out, or contact forms fail but JSON posts work (the check only applies to form content types)

Not for this skill when

• JSON or API posts fail (checkOrigin only inspects form-encoded POST, PATCH, PUT, DELETE)
• The site uses static output (checkOrigin only runs for SSR/on-demand pages)
• You want the quick fix of disabling checkOrigin (dont; see below)

Steps

1. Confirm the pattern: 403 on form POSTs in production, fine in local dev. That means Astro is rejecting a legitimate same-origin POST, not an actual attack. Behind a proxy, Astro ignores X-Forwarded-Host and X-Forwarded-Proto unless your domain is allowlisted, so the Origin header no longer matches the URL Astro reconstructs.
2. In astro.config, add your real production hostname (requires Astro 5.14.2 or later):

security: {
  allowedDomains: [{ hostname: 'YOUR_SITE_HOST', protocol: 'https' }]
}

Match the hostname and protocol the browser actually uses. Serving several subdomains? List each one explicitly, or use a wildcard pattern like **.YOUR_SITE_HOST.

3. Rebuild and redeploy, then resubmit the form. Expected: the POST returns 200 or 302 instead of 403.
4. On Astro below 5.14.2, allowedDomains does not exist: upgrade instead of hunting for another config knob.

Do not disable checkOrigin

Setting checkOrigin to false globally clears the error by removing CSRF protection from every form in the app. It hides the misconfiguration instead of fixing it.

Variant phrasings

403 Cross-site POST form submissions are forbidden

The exact message; same fix as above.

Every form POST fails in production with a CSRF 403

Same root cause: the proxy hides the real origin from Astro's middleware.

Sign-out or login form blocked behind a proxy

The most common shape of this error: auth forms POST in production and 403 while everything else looks fine.

Why it happens

Astro's checkOrigin middleware compares the browser's Origin header against the URL Astro reconstructs for the request. Behind a proxy, the reconstruction uses the internal scheme and host, the Origin no longer matches, and every form-encoded write is rejected as cross-site. JSON posts are unaffected because the check only applies to form content types.

Edge cases

• The allowlist entry [{}] trusts any forwarded host, which reopens host-header injection: reserve it for trusted dev setups.
• Only on-demand rendered (SSR) pages run the check; static output never triggers this error.

Related skills

• cross-site post form submissions are forbidden astro (alternate coverage): https://vectle.com/skills/skl_c8OUWMxAb88csiXGJb0PjA

Provenance

Verified against the official docs: https://docs.astro.build/en/reference/configuration-reference/