OPC UA Security

OPC UA (Open Platform Communications Unified Architecture) is the standard machine-to-machine protocol for industrial automation — the reference protocol of Industry 4.0/RAMI 4.0 — letting PLCs, SCADA systems, and devices from different vendors exchange data and control commands. Unlike legacy ICS protocols (Modbus, Profinet), it was designed with security: authentication, signing, and encryption are in the spec. The recurring failure is the gap between “built in” and “turned on” — 92% of internet-reachable servers show configuration issues and 44% allow unauthenticated access (Dahlmanns et al. 2020). ¹

Protocol shape (on the wire)

• Transport: binary opc.tcp on port 4840 (default). Also HTTPS bindings (4843/443) and vendor listeners: 49320 (KepServerEX), 62541 (OPC Foundation reference stack), 48050 (UaGateway). Multiple endpoints per host.
• Message types: HEL (hello), OPN (OpenSecureChannel), MSG, CLO.
• Endpoint model: each endpoint advertises a triple chosen independently:
  • SecurityMode: None / Sign / SignAndEncrypt
  • SecurityPolicy: crypto suite (Basic128Rsa15 and Basic256 are deprecated; floor is Basic256Sha256, modern is Aes128_Sha256_RsaOaep / Aes256_Sha256_RsaPss)
  • UserIdentityToken: Anonymous / Username&Password / Certificate
• Discovery is unauthenticated: FindServers, FindServersOnNetwork, GetEndpoints must work without security — they leak ApplicationUri, product/vendor strings, endpoints.
• Application authentication is certificate-based with a per-application Trustlist; user authentication is a separate layer on top. Anonymous user access is far less dangerous when application auth is enforced — many deployments disable the wrong layer.
• Address space: hierarchical nodes. Start browsing at ObjectsFolder (i=85). Useful NodeIds: i=2253 (Server — vendor strings), i=2256 (ServerStatus), i=2267 (ServerDiagnosticsSummary — session counts). ²

Why deployments fail (Erba et al. 2021, 48 artifacts)

38 of 48 products/libraries had security issues; 7 supported no security at all. The failures cluster in Trustlist (certificate) management — three recurring patterns:

1. Missing Trustlist support — signing/encryption offered but no cert validation → MitM always possible.
2. Trustlist disabled by default / auto-accept — e.g., node-opcua’s automaticallyAcceptUnknownCertificate=true; Siemens/Bachmann instructions default to accept-all. A “secure” connection that’s actually wide open.
3. Certificate exchange over unauthenticated OpenSecureChannel — vendor instructions (ABB, Beckhoff, Bosch Rexroth, Codesys, Eaton, GE, Hitachi, Lenze, Panasonic, Wago) tell users to push certs over the insecure channel then trust them → attacker installs a rogue client cert. Codesys propagation potentially touches 400+ manufacturers.

Demonstrated consequences: Rogue Server, Rogue Client, and Middleperson attacks that steal credentials (even across “encrypted” sessions), eavesdrop on process data, manipulate sensor/actuator values, and blind operators (screens show normal). The standard itself permits these insecure behaviors — partly a specification flaw.

Assessment playbook (detect / confirm / remediate)

• Discover: nmap -sV -Pn -n --open -p 4840,4843,49320,48050,53530,62541 <target>. Shodan: port:4840, product:"opc ua", ssl:"urn:opcua".
• Fingerprint: GetEndpoints per transport — capture SecurityPolicyUri, SecurityMode, UserTokenType. Any endpoint offering SecurityMode None or policy Basic128Rsa15 is a finding.
• Confirm anonymous access: connect with an anonymous UserIdentityToken, browse from i=85. If allowed, test Call on maintenance Method nodes (e.g., ns=2;s=Reset) — vendors often forget role bindings on custom methods.
• Confirm cert validation: connect with a self-signed throwaway cert — if the server accepts it, the Trustlist is disabled/auto-accept (the Erba et al. finding).
• Scan: opalopc -vv opc.tcp://<target>:4840 — flags anonymous logins, weak policies, cert validation errors, writable variables. Secura opcattack tests Basic128Rsa15 padding oracles and HTTPS reflection bypasses.
• Known CVEs to check: open62541 ≤1.4.6 (CVE-2024-53429, pre-auth UAF crash); OPC Foundation .NET stack <1.5.374.158 (CVE-2024-42512/13); Softing SDK (CVE-2025-7390, CN-replay cert bypass).

Remediation priorities

1. Kill SecurityMode None on anything carrying process data; enforce SignAndEncrypt.
2. Remove deprecated policies (Basic128Rsa15, Basic256); floor = Basic256Sha256.
3. Enforce the Trustlist — auto-accept off; root of trust established out-of-band, never via unauthenticated SecureChannel exchange. One cert per instance from a company CA (GDS-managed at scale); never trust a public CA root.
4. Enforce both authentication layers: application (certs) + user (no anonymous) with RBAC.
5. Segment; consider Reverse Connect (server dials out — OPC UA Part 6) so no inbound firewall ports stay open. Note Reverse Connect widens the client-side attack surface (rogue server → client RCE), so clients must validate ServerUri, EndpointUrl, and server certs strictly.
6. Keep auditing on and watched; patch the stack and gateways; verify config matches policy by testing (connect with an invalid cert and confirm rejection).

Cross-references

• ntlm-relay-attacks — the Windows-domain analogue: another protocol where security is optional-by-default and the fix is enforcement, not new crypto
• windows-server-2012-r2-security-assessment — likely co-present on the same engagement
• institutionally-constrained-technology-adoption — OT security debt as an institutional phenomenon: organizations rationally under-invest in enforcement
• cooperation-and-defection — vendor instruction quality as a norm-enforcement failure (Erba et al.’s “misleading instructions” finding)

See Also

• hermes-webui

Sources

• Alessandro Erba, Anne Muller, Nils Ole Tippenhauer (CISPA) 2021 — Security Analysis of Vendor Implementations of the OPC UA Protocol for Industrial Control Systems
• 4840 - Pentesting OPC UA (HackTricks)

Footnotes

1. Alessandro Erba, Anne Muller, Nils Ole Tippenhauer (CISPA) 2021 — Security Analysis of Vendor Implementations of the OPC UA Protocol for Industrial Control Systems ↩

2. 4840 - Pentesting OPC UA (HackTricks) ↩