Anonymous sessions used to short-circuit in HandleImpersonation — "let the SDK's
default validation stand" — so they always connected and always carried no roles.
Now admission and authority are separated:
admission -> OpcUa:AllowAnonymous, the HOST's decision (it owns token policies)
authority -> the authenticator, resolving the reserved principal "anonymous"
through the SAME grant path a key's principal goes through
So anonymous can be granted WRITE, not merely tolerated as read-only, and it
arrives at the downstream gates as the same RoleCarryingUserIdentity a key
produces — which is the real payoff: no gate carries an anonymous-shaped branch,
so there is no bypass path to get wrong.
AllowAnonymous defaults to TRUE on purpose. Defaulting it false would silently
deny every client without a key the moment this deploys, including ScadaBridge's
Data Connection Layer in another repo. Making keys mandatory is a rollout
decision, not an upgrade side effect. When false, the Anonymous token policy is
withheld so a client is refused at DISCOVERY rather than only at activation, and
activation is rejected independently in case a client attempts a token type the
endpoint never advertised.
Two fail-closed choices, both falsified:
- A failure to read anonymous's grants DENIES rather than admitting with no
roles. Admitting is the pre-#520 behaviour, and defaulting to it exactly when
the server cannot determine what anonymous may do stops being harmless once
reads are gated. Making it admit reds Anonymous_denies_when_grants_cannot_be_read.
- A key may not authenticate as the reserved "anonymous" principal, or a
credentialled client silently inherits whatever anonymous was granted.
AuthenticateAnonymousAsync is a DEFAULT interface implementation returning
admit-with-no-roles, i.e. exactly the pre-#520 behaviour, so
NullOpcUaUserAuthenticator and every test double keep working rather than being
forced to reimplement it — important because driver-only nodes run the Null one
and would otherwise black out. Note a default interface method is only reachable
through the interface-typed reference, which is how OpcUaApplicationHost holds it.
An existing test asserted the old fall-through and correctly went red; it is
rewritten to pin the new contract rather than deleted, with the reason recorded.
OpcUaServer 366, Host 42 green.
Claude-Session: https://claude.ai/code/session_015p7wGqy3YpZNCpDzTpGMKo
Joseph Doherty
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