Client token validation

openvpn-auth-oauth2 supports advanced token validation using the Common Expression Language (CEL). CEL allows you to write custom validation rules to verify that the OAuth2 ID token claims match the OpenVPN user's context.

Overview

CEL validation provides a flexible way to enforce security policies by allowing you to write custom expressions that evaluate to true or false. This validation happens:

1. During interactive authentication - After the OAuth2 authentication flow completes but before the OpenVPN connection is established
2. During token refresh - When an existing OpenVPN session is refreshed using a refresh token (non-interactive authentication)

This ensures that access policies are continuously enforced throughout the lifecycle of the VPN connection, not just during initial authentication.

Configuration

To enable CEL validation, configure the oauth2.validate.expression property in your configuration file:

YAML Configuration

oauth2:
  validate:
    expression: 'openVPNUserCommonName == oauth2TokenClaims.preferred_username'

Environment Variable

CONFIG_OAUTH2_VALIDATE_EXPRESSION='openVPNUserCommonName == oauth2TokenClaims.preferred_username'

Important

CEL validation is performed both during initial OAuth2 authentication and during token refresh. This means your validation rules will be continuously enforced throughout the entire lifecycle of a VPN session. Make sure your expressions account for both scenarios using the authMode variable if needed.

Available Variables

The following variables are available in your CEL expressions:

Variable	Type	Description
authMode	string	The authentication mode: "interactive" (initial OAuth2 login) or "non-interactive" (token refresh)
openVPNSessionState	string	The OpenVPN session state (e.g., "", "Empty", "Initial", "Authenticated", "Expired", "Invalid", "AuthenticatedEmptyUser", "ExpiredEmptyUser")
openVPNUserCommonName	string	The common name (CN) of the OpenVPN client certificate
openVPNUserIPAddr	string	The IP address of the OpenVPN client
oauth2TokenIPAddr	string	The IP address claim from the OAuth2 ID token
oauth2TokenClaims	map<string, dynamic>	All claims from the OAuth2 ID token

Expression Requirements

• The CEL expression must evaluate to a boolean (true or false)
• If the expression evaluates to true, the user is granted access
• If the expression evaluates to false, the user is denied access
• If the expression evaluation fails (e.g., syntax error, accessing a non-existent claim), the user is denied access

Safe Claim Access

Use the has() function to safely check for claim existence before accessing it:

oauth2:
  validate:
    expression: |
      has(oauth2TokenClaims.department) &&
      oauth2TokenClaims.department == 'engineering'

Important

If you try to access a claim that doesn't exist without using has(), the expression evaluation will fail, and the user will be denied access.

Examples

Basic Username Validation

Ensure the OpenVPN common name matches the OAuth2 username claim:

oauth2:
  validate:
    expression: 'openVPNUserCommonName == oauth2TokenClaims.preferred_username'

Email Domain Validation

Only allow users with email addresses from specific domains:

oauth2:
  validate:
    expression: |
      has(oauth2TokenClaims.email) &&
      oauth2TokenClaims.email.endsWith('@example.com')

Multiple Condition Validation

Combine multiple conditions with logical operators:

oauth2:
  validate:
    expression: |
      openVPNUserCommonName == oauth2TokenClaims.preferred_username &&
      has(oauth2TokenClaims.email_verified) &&
      oauth2TokenClaims.email_verified == true

Group-Based Validation

Allow access only if the user belongs to specific groups:

oauth2:
  validate:
    expression: |
      has(oauth2TokenClaims.groups) &&
      ('vpn-users' in oauth2TokenClaims.groups || 'administrators' in oauth2TokenClaims.groups)

IP Address Claim Validation

Validate that the VPN client IP matches the IP address claim from the token:

oauth2:
  validate:
    expression: 'openVPNUserIPAddr == oauth2TokenIPAddr'

Case-Insensitive Username Validation

Compare usernames in a case-insensitive manner using the lowerAscii() function:

oauth2:
  validate:
    expression: |
      has(oauth2TokenClaims.preferred_username) && openVPNUserCommonName.lowerAscii() == string(oauth2TokenClaims.preferred_username).lowerAscii()

Important

When accessing claims from oauth2TokenClaims that you want to use with string functions, you may need to cast them to string using string() since claims are stored as dynamic types.

Complex Custom Logic

Combine multiple conditions for sophisticated validation rules:

oauth2:
  validate:
    expression: |
      openVPNUserCommonName == oauth2TokenClaims.sub &&
      (
        (has(oauth2TokenClaims.role) && oauth2TokenClaims.role == 'admin') ||
        (has(oauth2TokenClaims.vpn_access) && oauth2TokenClaims.vpn_access == true)
      ) &&
      (!has(oauth2TokenClaims.account_locked) || oauth2TokenClaims.account_locked == false)

Email Prefix Validation

Extract and validate the prefix of an email address:

oauth2:
  validate:
    expression: |
      has(oauth2TokenClaims.email) &&
      string(oauth2TokenClaims.email).split('@')[0] == openVPNUserCommonName

Username Format Validation

Validate that a username contains only allowed characters using regular expression:

oauth2:
  validate:
    expression: |
      has(oauth2TokenClaims.preferred_username) &&
      string(oauth2TokenClaims.preferred_username).matches('^[a-zA-Z0-9._-]+$')

String Length Validation

Ensure usernames meet minimum length requirements:

oauth2:
  validate:
    expression: |
      openVPNUserCommonName.size() >= 3 &&
      has(oauth2TokenClaims.preferred_username) &&
      string(oauth2TokenClaims.preferred_username).size() >= 3

Domain-Based Routing

Allow different IP ranges based on email domain:

oauth2:
  validate:
    expression: |
      has(oauth2TokenClaims.email) &&
      (
        (string(oauth2TokenClaims.email).endsWith('@internal.company.com') &&
         openVPNUserIPAddr.startsWith('10.0.')) ||
        (string(oauth2TokenClaims.email).endsWith('@company.com') &&
         openVPNUserIPAddr.startsWith('192.168.'))
      )

Authentication Mode Based Validation

Apply different validation rules based on whether this is an initial login or a token refresh:

oauth2:
  validate:
    expression: |
      authMode == 'interactive' ||
      (authMode == 'non-interactive' && has(oauth2TokenClaims.refresh_allowed) && oauth2TokenClaims.refresh_allowed == true)

Session State Validation

Validate based on the current OpenVPN session state:

oauth2:
  validate:
    expression: |
      openVPNSessionState in ['Initial', 'Authenticated', 'AuthenticatedEmptyUser'] &&
      openVPNUserCommonName == oauth2TokenClaims.preferred_username

Combined Mode and State Validation

Combine authentication mode and session state for fine-grained control:

oauth2:
  validate:
    expression: |
      (authMode == 'interactive' && openVPNSessionState == 'Initial') ||
      (authMode == 'non-interactive' && openVPNSessionState == 'Authenticated')