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.
OPC Foundation SDK 2.0.1's SessionManager lockout refused EVERY
activation (Anonymous included, before the token is read) from a client
with 5 failed identity validations in 60 s, for 5 minutes, keyed on cert
thumbprint else peer IP. Disable it (MaxFailedAuthenticationAttempts=0
on built and loaded configs) and enforce FailedCredentialLockout in the
identity-registry adapter instead: UserName activations only, keyed on
thumbprint+peer together, same 5/60s/300s defaults, configurable via
OpcUa:CredentialLockout.
Joseph Doherty