Apache PLC4X versions from 0.9.0 up to but not including 1.0.0 contain a cryptographic verification flaw in the OPC UA driver. In versions 0.9.0 through 0.11.0, message signature failures are logged but not enforced, and server certificates are accepted from unauthenticated discovery responses. Versions 0.12.0 through 0.13.1 invert signature validation logic and accept server certificates without trust anchors by default. Across all affected versions, the default security policy is 'None', and from 0.12.0 onward, the driver silently downgrades to weaker security policies than configured, with endpoint selection favoring the weakest matching endpoint starting in 0.13.0. These flaws allow an attacker positioned between client and server to impersonate the server and compromise secure-channel communications. Version 1.0.0 corrects these issues by properly verifying signatures, enforcing certificate trust validation, defaulting to a strong security policy (Basic256Sha256 with SignAndEncrypt), and failing connections if negotiated policies are weaker than configured.

An attacker with network access between the client and server can impersonate the OPC UA server, enabling interception, forgery, or modification of secure-channel traffic, including sensitive user credentials. This compromises confidentiality and integrity of communications within the affected Apache PLC4X OPC UA driver versions.

A fix is available in Apache PLC4X version 1.0.0. Users are strongly advised to upgrade to version 1.0.0, which correctly enforces message signature verification, validates server certificates against a configured trust store or pinned certificate, defaults to a strong security policy (Basic256Sha256 with SignAndEncrypt), and rejects connections with weaker negotiated security policies. No alternative mitigations are indicated.