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We want to apply for HSTS preload.

HSTS preload is hardcore: once on, you can't fall back to HTTP without months of browser warnings. Only enable when every subdomain is HTTPS-only.

Try this first

  1. 1Verify the apex, www variant and all subdomains run HTTPS without errors. No mixed content, no expired certs.
  2. 2Add the HSTS header via your web server or Cloudflare: Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
  3. 3Run the header at a short max-age first (e.g. 86400, one day) for a couple of months. You can still back out without damage.
  4. 4No issues? Set max-age to 31536000 (a year) and keep that for a week or two.
  5. 5Submit the domain at hstspreload.org. Google's list leads; other browsers follow.
  6. 6Document HSTS preload is active: rollback takes months to a year as browsers cache the rule.

When to bring us in

Operating for an org where HTTP fallback could ever be needed (legacy systems, IoT on subdomains)? Don't enable HSTS preload without a security engineer who weighed the impact.

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