change(security): add Fetch Metadata request headers - #196
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Browsers have sent Sec-Fetch-Site / -Mode / -Dest on every request since Safari 16.4 completed support in March 2023, and OWASP now names them a primary CSRF defence. Nothing in the spec covered them beyond a passing mention on cookie-attributes. Status is `recommended`: the platform contract does not break without a resource isolation policy, and SameSite plus CSRF tokens remain the alternatives it sits alongside. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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What changed
New spec page: Fetch Metadata request headers —
/spec/security/fetch-metadata/,status: recommended,order: 63.Also in this PR:
relatedSlugswired oncookie-attributes,frame-ancestorsandcross-origin-isolation.2026-09-04-fetch-metadata.md(added).public/og/spec/security/fetch-metadata.pngplus the four count-driven images).npm run sign:skill— SKILL.md page count 168 → 169, digest re-signed.Why now
The spec had no page for
Sec-Fetch-Site/-Mode/-Dest— only a one-clause mention on cookie-attributes as a CSRF fallback. Support has been complete since Safari 16.4 (March 2023), and OWASP's CSRF cheat sheet now leads with Fetch Metadata rather than listing it as an extra, so this is a mature convention the spec was simply missing rather than an emerging one.It passes the auditable-outcome test cleanly: you can send a request with
Sec-Fetch-Site: cross-sitefrom outside and observe whether the server refuses it.Primary sources
Sec--prefixed names unforgeable from script (cited inline).Status justification
recommended, notrequired. The web platform contract does not break without a resource isolation policy, andSameSitecookies plus CSRF tokens remain viable on their own — the page says explicitly that this is defence in depth alongside them, not a replacement. Notoptionaleither: OWASP naming it a primary defence, on universal browser support, puts it above "depends on context".Browser-support nuance
BCD (via MDN MCP) shows
Sec-Fetch-Site/-Mode/-Destavailable everywhere from Safari 16.4, butSec-Fetch-Userhas never shipped in Safari — which is why webstatus.dev reports thefetch-metadataweb-feature as Baselinelimited. The page calls this out twice (in## What it isand## Common mistakes): a rule that requiresSec-Fetch-Userlocks out every Safari visitor. The three headers a policy actually gates on are fine.Not shipped here
The page carries the "specced, deliberately not shipped" note per CLAUDE.md. This site is static and cookieless — no session to ride, no state-changing endpoint — and the one endpoint that accepts
POST,/reports, is supposed to receive requests the user never initiated.Verification
npm run build,npm run lint,npm run format:checkandnpm run check:skillall pass. Every inline/spec/...link in the new page resolves againstdist/.Draft — not for merge without review.
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