GHSA-p279-2cqp-84jg: OpenDJ SASL PLAIN authzid bypassing the proxy ACI scope check

Summary When a SASL PLAIN bind supplies an authorization identity (authzid) that resolves to a different user, PlainSASLMechanismHandler verified only the PROXIED_AUTH privilege and never evaluated the "proxy" access-control right (the mayProxy ACI scope check). As a result, any account holding the proxied-auth privilege could assume any resolvable non-root identity without being granted a proxy ACI for that target. This diverges from every other proxy path in OpenDJ — the proxied-authorization controls (RFC 4370) and the DIGEST-MD5 / GSSAPI authzid handlers all require both the privilege and the mayProxy scope grant. Impact Privilege escalation / authorization bypass: a holder of proxied-auth can act as arbitrary directory users beyond the scope intended by the deployment's proxy ACIs, defeating the ACI-based restriction on *which* identities may be impersonated. Root/Directory Manager is not assumable this way. Fix Enforce the mayProxy scope check on the SASL PLAIN authzid path (both dn: and u:/bare forms), sharing one hasProxyAccess helper with the DIGEST-MD5/GSSAPI path. Denial returns INVALID_CREDENTIALS (49) before password verification — matching DIGEST-MD5/GSSAPI — so an unauthenticated client cannot distinguish a missing privilege from a missing ACI grant. Workaround Restrict or revoke the proxied-auth privilege until upgraded.