How does OpenVPN work?

How OpenVPN builds a secure tunnel

OpenVPN works by creating a virtual network path between a client and a VPN server, then encrypting traffic so it can cross untrusted networks without exposing the contents in transit. The client and server first establish trust, then negotiate session keys, and finally move traffic through the tunnel as if the remote network were locally reachable.

That design matters because OpenVPN is not just “encrypted internet access.” It is a transport and access-control mechanism: it decides who can connect, what routes the client receives, and how much of the network becomes reachable once the tunnel is up. That is why certificate handling, credential strength, and route policy are part of how it works, not add-ons.

For deployment context, the VPN Security Guide is useful because it covers the operational realities that shape VPN behaviour, including hardening, split tunnelling, patching, and VPN replacement decisions.

Authentication, routing, and policy after connection

OpenVPN commonly authenticates clients with certificates, usernames and passwords, or both, depending on how the server is configured. Once authentication succeeds, the server can assign routes, push DNS settings, and define whether the client gets full tunnel access or only selected destinations. In practice, that means the security posture depends as much on server policy as on the encryption layer.

OpenVPN also supports policy choices that change exposure. A full-tunnel setup forces most traffic through the VPN, while split tunnelling sends only selected traffic through the tunnel and leaves the rest on the local network path. Full tunnelling reduces direct exposure to untrusted networks, while split tunnelling can reduce latency but increases the number of paths an attacker may use if the endpoint is compromised.

That is why administrators should think about route scope, authentication strength, and key management together. A well-configured tunnel can still create broad internal reach if the client is overtrusted, the routes are too permissive, or the server treats a successful login as blanket trust.

What usually goes wrong in real deployments

The main failure modes are credential theft, weak certificate handling, excessive route access, and poor separation between user traffic and internal resources. If a client credential or private key is stolen, an attacker may be able to establish a legitimate-looking tunnel and then move laterally through the network that the VPN exposes. If split tunnelling is overly broad, the VPN can become a bridge into systems that should remain segmented.

OpenVPN itself is only one part of the trust chain. The server, client software, certificate authority material, route policies, and endpoint hardening all affect whether the tunnel provides controlled access or simply creates a convenient remote entry point. Current guidance generally treats VPNs as a boundary control that needs tight privilege, short-lived secrets where possible, and careful monitoring rather than as a complete trust solution.

For identity and privilege governance, the relevant control question is not only “did the user authenticate?” but also “what access did that authentication unlock?” Broad network routes, long-lived credentials, and unattended sessions increase the blast radius of any compromise.

Risk and Threat Considerations

OpenVPN concentrates trust into a small number of authentication and routing decisions, so a single stolen credential, private key, or overly broad client profile can open far more of the network than the user should reach.

Failure mechanism: Attackers abuse valid tunnel access, weak client key protection, or permissive routes to enter the internal network through a channel that looks legitimate to perimeter controls.

Impact: The result can be unauthorized access to internal services, lateral movement, and exposure of systems that would otherwise be isolated from the public internet.

Practitioner Guidance

What to verify: Confirm which authentication method is in use, what routes are pushed to the client, and whether split tunnelling is intentionally enabled. If a profile grants access beyond the user’s actual need, treat that as an access-control problem, not just a VPN configuration choice.

What to prioritise: Protect private keys, rotate credentials when compromise is suspected, and review dormant or shared VPN profiles first. Where possible, make access decisions as narrow as the business use case allows, because the practical risk comes from tunnel scope as much as from tunnel encryption.

Practitioner takeaway: OpenVPN is secure only to the extent that its authentication, keys, and route policy are controlled tightly; the tunnel itself is protective, but the access it grants is what usually determines the real risk.

Related resources from NHI Mgmt Group

• How does packet capture (PCAP) work?
• How does Firewall as a Service (FWaaS) work?
• How does Security Service Edge (SSE) work?
• How does Suricata work?