A legitimate consumer opening several keyed sessions at once from one key
(e.g. a ScadaBridge site node behind one NAT IP on a None endpoint) must
never see BadUserAccessDenied for parallelism. BeginAsync now hands out at
most MaxFailedAttempts - recordedFailures verification slots per key;
further activations queue FIFO and are woken as slots free. A waiter is
refused only when the key locks out (never reaching the authenticator) or,
uncounted, after OpcUa:CredentialLockout:SlotWaitSeconds (default 10,
BadTimeout). Cancelled waiters are dequeued; pruning accounts for waiters.
The security property is unchanged: no more than MaxFailedAttempts
unverified guesses in progress-or-failed per window.
- Reserve an in-flight slot per client before authenticating (TryBegin /
Complete / Abandon under one lock); refuse once recorded failures +
in-flight attempts reach MaxFailedAttempts, so a parallel burst cannot
pass the check before its first failure is recorded.
- An expired lockout resets the count (LockoutSeconds < FailureWindowSeconds
relocked on every single failure after the first lockout).
- Validate OpcUa:CredentialLockout at host start: disabled, or both
durations > 0.
- Log the first refusal of a lockout episode at Information, the rest at
Debug.
- Document that on None endpoints the key is effectively IP-only (NAT
neighbours share a UserName count) and a null peer shares one key.
Joseph Doherty