lmxopcua/docs/security.md

Transport Security

Overview

The LmxOpcUa server supports configurable transport security profiles that control how data is protected on the wire between OPC UA clients and the server.

There are two distinct layers of security in OPC UA:

• Transport security -- secures the communication channel itself using TLS-style certificate exchange, message signing, and encryption. This is what the Security configuration section controls.
• UserName token encryption -- protects user credentials (username/password) sent during session activation. The OPC UA stack encrypts UserName tokens using the server's application certificate regardless of the transport security mode. This means UserName authentication works on None endpoints too — the credentials themselves are always encrypted. However, a secure transport profile adds protection against message-level tampering and eavesdropping of data payloads.

Supported Security Profiles

The server supports seven transport security profiles:

Profile Name	Security Policy	Message Security Mode	Description
None	None	None	No signing or encryption. Suitable for development and isolated networks only.
Basic256Sha256-Sign	Basic256Sha256	Sign	Messages are signed but not encrypted. Protects against tampering but data is visible on the wire.
Basic256Sha256-SignAndEncrypt	Basic256Sha256	SignAndEncrypt	Messages are both signed and encrypted. Full protection against tampering and eavesdropping.
Aes128_Sha256_RsaOaep-Sign	Aes128_Sha256_RsaOaep	Sign	Modern profile with AES-128 encryption and SHA-256 signing.
Aes128_Sha256_RsaOaep-SignAndEncrypt	Aes128_Sha256_RsaOaep	SignAndEncrypt	Modern profile with AES-128 encryption. Recommended for production.
Aes256_Sha256_RsaPss-Sign	Aes256_Sha256_RsaPss	Sign	Strongest profile with AES-256 and RSA-PSS signatures.
Aes256_Sha256_RsaPss-SignAndEncrypt	Aes256_Sha256_RsaPss	SignAndEncrypt	Strongest profile. Recommended for high-security deployments.

Multiple profiles can be enabled simultaneously. The server exposes a separate endpoint for each configured profile, and clients select the one they prefer during connection.

If no valid profiles are configured (or all names are unrecognized), the server falls back to None with a warning in the log.

Configuration

Transport security is configured in the Security section of appsettings.json:

{
  "Security": {
    "Profiles": ["None"],
    "AutoAcceptClientCertificates": true,
    "RejectSHA1Certificates": true,
    "MinimumCertificateKeySize": 2048,
    "PkiRootPath": null,
    "CertificateSubject": null
  }
}

Properties

Property	Type	Default	Description
Profiles	string[]	["None"]	List of security profile names to expose as server endpoints. Valid values: None, Basic256Sha256-Sign, Basic256Sha256-SignAndEncrypt, Aes128_Sha256_RsaOaep-Sign, Aes128_Sha256_RsaOaep-SignAndEncrypt, Aes256_Sha256_RsaPss-Sign, Aes256_Sha256_RsaPss-SignAndEncrypt. Profile names are case-insensitive. Duplicates are ignored.
AutoAcceptClientCertificates	bool	true	When true, the server automatically trusts client certificates that are not already in the trusted store. Set to false in production for explicit trust management.
RejectSHA1Certificates	bool	true	When true, client certificates signed with SHA-1 are rejected. SHA-1 is considered cryptographically weak.
MinimumCertificateKeySize	int	2048	Minimum RSA key size (in bits) required for client certificates. Certificates with shorter keys are rejected.
PkiRootPath	string?	null (defaults to %LOCALAPPDATA%\OPC Foundation\pki)	Override for the PKI root directory where certificates are stored. When null, uses the OPC Foundation default location.
CertificateSubject	string?	null (defaults to CN={ServerName}, O=ZB MOM, DC=localhost)	Override for the server certificate subject name. When null, the subject is derived from the configured ServerName.

Example: Development (no security)

{
  "Security": {
    "Profiles": ["None"],
    "AutoAcceptClientCertificates": true
  }
}

Example: Production (encrypted only)

{
  "Security": {
    "Profiles": ["Basic256Sha256-SignAndEncrypt"],
    "AutoAcceptClientCertificates": false,
    "RejectSHA1Certificates": true,
    "MinimumCertificateKeySize": 2048
  }
}

Example: Mixed (sign and encrypt endpoints, no plaintext)

{
  "Security": {
    "Profiles": ["Basic256Sha256-Sign", "Basic256Sha256-SignAndEncrypt"],
    "AutoAcceptClientCertificates": false
  }
}

PKI Directory Layout

The server stores certificates in a directory-based PKI store. The default root is:

%LOCALAPPDATA%\OPC Foundation\pki\

This can be overridden with the PkiRootPath setting. The directory structure is:

pki/
  own/        Server's own application certificate and private key
  issuer/     CA certificates that issued trusted client certificates
  trusted/    Explicitly trusted client (peer) certificates
  rejected/   Certificates that were presented but not trusted

Certificate Trust Flow

When a client connects using a secure profile (Sign or SignAndEncrypt), the following trust evaluation occurs:

1. The client presents its application certificate during the secure channel handshake.
2. The server checks whether the certificate exists in the trusted/ store.
3. If found, the connection proceeds (subject to key size and SHA-1 checks).
4. If not found and AutoAcceptClientCertificates is true, the certificate is automatically copied to trusted/ and the connection proceeds.
5. If not found and AutoAcceptClientCertificates is false, the certificate is copied to rejected/ and the connection is refused.
6. Regardless of trust status, the certificate must meet the MinimumCertificateKeySize requirement and pass the SHA-1 check (if RejectSHA1Certificates is true).

On first startup with a secure profile, the server automatically generates a self-signed application certificate in the own/ directory if one does not already exist.

Production Hardening

The default settings prioritize ease of development. Before deploying to production, apply the following changes:

1. Disable automatic certificate acceptance

Set AutoAcceptClientCertificates to false so that only explicitly trusted client certificates are accepted:

{
  "Security": {
    "AutoAcceptClientCertificates": false
  }
}

After changing this setting, you must manually copy each client's application certificate (the .der file) into the trusted/ directory.

2. Remove the None profile

Remove None from the Profiles list to prevent unencrypted connections:

{
  "Security": {
    "Profiles": ["Aes256_Sha256_RsaPss-SignAndEncrypt"]
  }
}

3. Configure LDAP authentication

Enable LDAP authentication to validate credentials against the GLAuth server. LDAP group membership controls what each user can do (read, write, alarm acknowledgment). See Configuration Guide for the full LDAP property reference.

{
  "Authentication": {
    "AllowAnonymous": false,
    "AnonymousCanWrite": false,
    "Ldap": {
      "Enabled": true,
      "Host": "localhost",
      "Port": 3893,
      "BaseDN": "dc=lmxopcua,dc=local",
      "ServiceAccountDn": "cn=serviceaccount,dc=lmxopcua,dc=local",
      "ServiceAccountPassword": "serviceaccount123"
    }
  }
}

While UserName tokens are always encrypted by the OPC UA stack (using the server certificate), enabling a secure transport profile adds protection against message-level tampering and data eavesdropping.

4. Review the rejected certificate store

Periodically inspect the rejected/ directory. Certificates that appear here were presented by clients but were not trusted. If you recognize a legitimate client certificate, move it to the trusted/ directory to grant access.

X.509 Certificate Authentication

The server supports X.509 certificate-based user authentication in addition to Anonymous and UserName tokens. When any non-None security profile is configured, the server advertises UserTokenType.Certificate in its endpoint descriptions.

Clients can authenticate by presenting an X.509 certificate. The server extracts the Common Name (CN) from the certificate subject and assigns the AuthenticatedUser and ReadOnly roles. The authentication is logged with the certificate's CN, subject, and thumbprint.

X.509 authentication is available automatically when transport security is enabled -- no additional configuration is required.

Audit Logging

The server generates audit log entries for security-relevant operations. All audit entries use the AUDIT: prefix and are written to the Serilog rolling file sink for compliance review.

Audited events:

• Authentication success: Logs username, assigned roles, and session ID
• Authentication failure: Logs username and session ID
• X.509 authentication: Logs certificate CN, subject, and thumbprint
• Certificate validation: Logs certificate subject, thumbprint, and expiry for all validation events (accepted or rejected)
• Write access denial: Logged by the role-based access control system when a user lacks the required role

Example audit log entries:

AUDIT: Authentication SUCCESS for user admin with roles [ReadOnly, WriteOperate, AlarmAck] session abc123
AUDIT: Authentication FAILED for user baduser from session def456
X509 certificate authenticated: CN=ClientApp, Subject=CN=ClientApp,O=Acme, Thumbprint=AB12CD34

CLI Examples

The Client CLI supports the -S (or --security) flag to select the transport security mode when connecting. Valid values are none, sign, encrypt, and signandencrypt.

Connect with no security

dotnet run --project src/ZB.MOM.WW.OtOpcUa.Client.CLI -- connect -u opc.tcp://localhost:4840/LmxOpcUa -S none

Connect with signing

dotnet run --project src/ZB.MOM.WW.OtOpcUa.Client.CLI -- connect -u opc.tcp://localhost:4840/LmxOpcUa -S sign

Connect with signing and encryption

dotnet run --project src/ZB.MOM.WW.OtOpcUa.Client.CLI -- connect -u opc.tcp://localhost:4840/LmxOpcUa -S encrypt

Browse with encryption and authentication

dotnet run -- browse -u opc.tcp://localhost:4840/LmxOpcUa -S encrypt -U operator -P secure-password -r -d 3

Read a node with signing

dotnet run -- read -u opc.tcp://localhost:4840/LmxOpcUa -S sign -n "ns=2;s=TestMachine_001/Speed"

The CLI tool auto-generates its own client certificate on first use (stored under %LOCALAPPDATA%\OpcUaCli\pki\own\). When connecting to a server with AutoAcceptClientCertificates set to false, you must copy the CLI tool's certificate into the server's trusted/ directory before the connection will succeed.

Troubleshooting

Certificate trust failure

Symptom: The client receives a BadSecurityChecksFailed or BadCertificateUntrusted error when connecting.

Cause: The server does not trust the client's certificate (or vice versa), and AutoAcceptClientCertificates is false.

Resolution:

1. Check the server's rejected/ directory for the client's certificate file.
2. Copy the .der file from rejected/ to trusted/.
3. Retry the connection.
4. If the server's own certificate is not trusted by the client, copy the server's certificate from pki/own/certs/ to the client's trusted store.

Endpoint mismatch

Symptom: The client receives a BadSecurityModeRejected or BadSecurityPolicyRejected error, or reports "No endpoint found with security mode...".

Cause: The client is requesting a security mode that the server does not expose. For example, the client requests SignAndEncrypt but the server only has None configured.

Resolution:

1. Verify the server's configured Profiles in appsettings.json.
2. Ensure the profile matching the client's requested mode is listed (e.g., add Basic256Sha256-SignAndEncrypt for encrypted connections).
3. Restart the server after changing the configuration.
4. Use the CLI tool to verify available endpoints:

   dotnet run -- connect -u opc.tcp://localhost:4840/LmxOpcUa -S none
   

   The output displays the security mode and policy of the connected endpoint.

Server certificate not generated

Symptom: The server logs a warning about application certificate check failure on startup.

Cause: The pki/own/ directory may not be writable, or the certificate generation failed.

Resolution:

1. Ensure the service account has write access to the PKI root directory.
2. Check that the PkiRootPath (if overridden) points to a valid, writable location.
3. Delete any corrupt certificate files in pki/own/ and restart the server to trigger regeneration.

SHA-1 certificate rejection

Symptom: A client with a valid certificate is rejected, and the server logs mention SHA-1.

Cause: The client's certificate was signed with SHA-1, and RejectSHA1Certificates is true (the default).

Resolution:

• Regenerate the client certificate using SHA-256 or stronger (recommended).
• Alternatively, set RejectSHA1Certificates to false in the server configuration (not recommended for production).

---

LDAP Authentication

The server supports LDAP-based user authentication via GLAuth (or any standard LDAP server). When enabled, OPC UA UserName token credentials are validated by LDAP bind. LDAP group membership is resolved once during authentication and mapped to custom OPC UA role NodeIds in the urn:zbmom:lmxopcua:roles namespace. These role NodeIds are stored on the session's RoleBasedIdentity.GrantedRoleIds and checked directly during write and alarm-ack operations.

Architecture

OPC UA Client → UserName Token → LmxOpcUa Server → LDAP Bind (validate credentials)
                                                  → LDAP Search (resolve group membership)
                                                  → Map groups to OPC UA role NodeIds
                                                  → Store on RoleBasedIdentity.GrantedRoleIds
                                                  → Permission checks via GrantedRoleIds.Contains()

LDAP Groups and OPC UA Permissions

All authenticated LDAP users can browse and read nodes regardless of group membership. Groups grant additional permissions:

LDAP Group	Permission
ReadOnly	No additional permissions (read-only access)
WriteOperate	Write FreeAccess and Operate attributes
WriteTune	Write Tune attributes
WriteConfigure	Write Configure attributes
AlarmAck	Acknowledge alarms

Users can belong to multiple groups. The admin user in the default GLAuth configuration belongs to all groups.

Effective Permission Matrix

The effective permission for a write operation depends on two factors: the user's session role (from LDAP group membership or anonymous access) and the Galaxy attribute's security classification. The security classification controls the node's AccessLevel — attributes classified as SecuredWrite, VerifiedWrite, or ViewOnly are exposed as read-only nodes regardless of the user's role. For writable classifications, the required write role depends on the classification.

	FreeAccess	Operate	SecuredWrite	VerifiedWrite	Tune	Configure	ViewOnly
Anonymous (AnonymousCanWrite=true)	Write	Write	Read	Read	Write	Write	Read
Anonymous (AnonymousCanWrite=false)	Read	Read	Read	Read	Read	Read	Read
ReadOnly	Read	Read	Read	Read	Read	Read	Read
WriteOperate	Write	Write	Read	Read	Read	Read	Read
WriteTune	Read	Read	Read	Read	Write	Read	Read
WriteConfigure	Read	Read	Read	Read	Read	Write	Read
AlarmAck (only)	Read	Read	Read	Read	Read	Read	Read
Admin (all groups)	Write	Write	Read	Read	Write	Write	Read

All roles can browse and read all nodes. The "Read" entries above mean the node is either read-only by classification or the user lacks the required write role. "Write" means the write is permitted by both the node's classification and the user's role.

Alarm acknowledgment is an independent permission controlled by the AlarmAck role and is not affected by security classification.

GLAuth Setup

The project uses GLAuth v2.4.0 as the LDAP server, installed at C:\publish\glauth\. See C:\publish\glauth\auth.md for the complete user/group reference and service management commands.

Configuration

Enable LDAP in appsettings.json under Authentication.Ldap. See Configuration Guide for the full property reference.

Security Considerations

• LDAP credentials are transmitted in plaintext over the OPC UA channel unless transport security is enabled. Use Basic256Sha256-SignAndEncrypt for production deployments.
• The GLAuth LDAP server itself listens on plain LDAP (port 3893). Enable LDAPS in glauth.cfg for environments where LDAP traffic crosses network boundaries.
• The service account password is stored in appsettings.json. Protect this file with appropriate filesystem permissions.