Challenge/Response Authentication in Access Server
OpenVPN's Access Server supports implementing challenge/response authentication in addition to username/password.
Access Server supports challenge-response authentication via a post-authentication script (PAS). This allows administrators to introduce an interactive authentication step in which users must respond to a dynamically generated challenge before the VPN session is established.
With challenge/response authentication enabled, Access Server:
Prompts the user with a challenge during login.
Evaluates the user's response using custom logic defined in a Python 3 PAS.
Allows or denies the VPN connection based on the validation result.
This mechanism can be used to:
Integrate with external verification systems.
Implement custom multi-factor authentication workflows.
Enforce additional security checks beyond standard credentials.
Challenge/response logic is executed during the post-authentication hook, before the VPN session is fully established.
Important
Challenge/response authentication is implemented through a custom post-authentication script. The administrator is responsible for designing, testing, and maintaining the validation logic. Improper implementation may prevent legitimate users from connecting or introduce security risks.