The public /v1/oauth/:provider/start route read handoff.attemptId
straight out of the request body and folded it into the signed state
token unconditionally. Since the private command plane's beginAuth
mints an attempt handle server-side and later redeems by that handle
(consumeAuthCompletionByAttempt), anyone who learned a pending handle
could carry it through their own OAuth flow and bind their own
identity to it -- the app's completeAuth(H) would then authenticate
the victim's session as the attacker. Same family as the takeover
chain closed in the prior pass: a caller-chosen identifier trusted as
authority.
normalizeOAuthHandoff now always strips attemptId from a
browser_completion handoff; a public caller has no attempt to
correlate and gets its completion code directly. The only path an
attempt handle can reach the state token is startOAuthFlow's new
trustedBrowserAttemptId option, which is a TypeScript function
argument set by beginAuth from the handle it just minted -- never a
value parsed out of a request body.
Also adds the uniqueness constraint the by-attempt redemption needed:
a partial unique index on (app_slug, attempt_id) WHERE attempt_id IS
NOT NULL AND consumed_at IS NULL, mirrored in InMemoryStore. Without
it, consumeAuthCompletionByAttempt's UPDATE could match more than one
row and silently discard all but the one it returned.
Flips Soul's attempt-handle-binding it.fails to a passing it.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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