docs(adr-0142): disclose the trust-on-first-use window A.4 left out

[wshallwshall]

### A.4 Residual (stated, not hidden) states only the availability edge — a legitimately reassigned username is permanently refused with no rebind path. Read on its own, it implies the takeover half is closed. It isn't, and the undisclosed half is the more consequential one.

The guard is trust-on-first-use

Its precondition is that the resolved account already carries a binding. AC-12 says so in terms:

WHEN the account is unbound, it SHALL record the binding on that login

So on an account that has never federated-logged-in there is nothing to compare against, the check short-circuits, and the binding is recorded for whoever arrives first.

That is exactly the scenario A.1 names as the gap this amendment closes — an IdP reassigns an already-allowed username to a different principal, the domain check passes, every ladder rung is green, and a session is minted on the prior person's account. A.1's own attack still succeeds against any unbound account, and the guard then protects the new holder's binding against everyone else.

Reproduced, not reasoned

Against shipped code (in-memory SQLite, RS256 id_token, stubbed token exchange, fake LDAP): an account created by AD-password login (oidc_subject NULL) was taken over by a different verified subject presenting the same reassigned preferred_username — same user row, session minted, the incumbent's LDAP-derived roles inherited — and a third subject was then refused.

The window is not narrow

Unbound is the default state:

• every account predating enabling federation — the entire existing population on day one;
• every account created afterwards by AD-password or Kerberos login, since those callers pass None and write no binding (A.2).

What bounds it: the attacker must be a principal the pinned IdP will mint a token for, within an allow-listed UPN suffix, and the target AD object must exist (hybrid-only). The suffix allow-list constrains the suffix, never the local part, and defaults to the configured ad_domain — the very suffix the organisation's own IdP mints. So the residual is conditional on IdP tenancy, not on engine configuration.

No decision is made here

The two follow-ons — refuse unbound accounts that predate federation, or an operator pre-binding step — both carry a rollout cost (lockout vs a new admin surface). That's an owner call, not a documentation one. This PR records the trade instead of half of it.

Worth noting (b) is the same admin surface A.4's existing availability follow-on already recommends, so one control can discharge both.

Downstream

Assessed as ASVS 10.5.2 (partial — a refusing control exists and ships on, but covers only bound accounts) and 6.8.1 (partial — cross-IdP identity is still keyed on the reassignable username, with (issuer, sub) able only to veto the row that claim already selected). Neither can reach pass while this window is open.

Documentation only — no code, no behaviour change. Cross-checked that the insert does not disturb the downstream scorecard's three anchors into this file: all sit above the insertion point and still resolve.

Third of three from the same review: #293 (TOTP clamp docs), #295 (anchor drift vs invalidation).

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