Why does VPN Split Tunneling matter for security teams?

Why split tunneling changes the security boundary

Split tunneling is not just a performance setting, it decides which traffic is forced through enterprise controls and which traffic escapes them. That means the same laptop can simultaneously sit inside the corporate trust boundary for some sessions and outside it for others. For security teams, that boundary choice affects inspection, logging, DNS resolution, and whether a compromise can pivot through the tunnel or around it.

When routing is forced through the VPN, security tooling has a better chance to inspect traffic and apply policy consistently. When selected traffic bypasses the VPN, users may gain speed and reduce congestion, but the organisation also accepts a second path that can be harder to observe and govern. That is why split tunneling is an architecture decision, not merely a user experience preference.

For teams formalising remote-access policy, the closest control question is whether the path to internal resources remains bounded and verifiable under a Zero Trust Architecture model. Split tunneling pushes you to define which traffic must be continuously evaluated, and which risks you are willing to leave outside the enterprise inspection plane.

What security teams lose when traffic bypasses the tunnel

The most practical downside is inconsistent visibility. If some traffic never reaches the corporate network, then web filtering, DLP, proxy logging, and certain threat detection workflows may never see it. That creates blind spots for malware callbacks, credential theft, unsafe web browsing, and exfiltration channels that would otherwise have been easier to monitor.

DNS handling is often where the weakness becomes visible first. If a device uses local DNS for Internet traffic while using enterprise DNS for internal names, teams need to know exactly where name resolution occurs, because that affects split-horizon behaviour, filtering, and the ability to detect unusual lookups. Routing mistakes can also send internal destinations over the wrong path or expose internal resolution patterns to networks the organisation does not control.

It is also worth reviewing the trust boundary around the endpoint itself. A device that can reach both the Internet directly and internal systems through a VPN can become a bridge if it is compromised, which is why policy should consider route design, endpoint hardening, and access scope together rather than separately. Remote Access Identity Guide is useful here because it frames VPN choice as part of a broader remote-access control model, not a standalone network setting.

How to decide whether split tunneling is acceptable

The right answer usually depends on what the VPN is protecting and what the endpoint is allowed to do. Split tunneling is easier to justify when users need direct Internet access for high-volume, low-risk activity and when internal applications are already protected by strong authentication, device checks, and least-privilege access. It is harder to justify when the same device handles sensitive internal data, admin functions, or third-party access.

Security teams should test the configuration the way an attacker or failure would experience it: can the device reach internal services while also reaching untrusted external services, can DNS be forced outside policy, and can a local application silently bypass the tunnel? Those questions matter because misconfiguration often creates the real exposure, not the feature itself. When in doubt, a tighter route model is usually easier to monitor and explain than a permissive one.

Where VPNs remain part of the design, teams should compare the split-tunnel posture with a more explicit access model such as VPN Security Guide, which covers split tunnelling, full tunnelling, appliance hardening, and replacement options. The practical decision is whether the VPN is still the best control for the trust boundary you actually need.

Risk and Threat Considerations

Split tunneling increases the chance that a compromised endpoint can communicate both inside and outside the enterprise under different policy paths. That creates a larger attack surface for data leakage, command-and-control traffic, and lateral movement attempts that may evade controls that only see tunnelled sessions.

Failure mechanism: Traffic that bypasses the VPN can escape central inspection, while DNS or routing inconsistencies can expose internal lookups, weaken monitoring, or let malware use the local Internet path for exfiltration and external contact.

Impact: Security teams may lose visibility into important endpoint activity, miss indicators of compromise, and allow a compromised device to bridge corporate and non-corporate networks with less friction than intended.

Practitioner Guidance

What to verify: Confirm which traffic classes are tunnelled, which are excluded, and whether DNS, proxy, and split-routes behave consistently on every supported client platform. A policy that exists only on paper is not enough; the device has to resolve names and reach destinations the way the design assumes.

Decision rule: If the device can access sensitive internal systems or privileged tools, default toward tighter tunnelling and stronger endpoint controls. If split tunneling is retained for usability or bandwidth reasons, treat it as a monitored exception and narrow the allowed bypass traffic as much as possible.

Practitioner takeaway: Split tunneling is acceptable only when the organisation can clearly state which traffic is outside the enterprise boundary, how that traffic is observed, and what prevents the endpoint from becoming a dual-homed risk.

Related resources from NHI Mgmt Group

• How should security teams decide whether JIT access is safe for non-human identities?
• How do security teams know whether split tunneling is still undermining CUI protection?
• Why does Web-Scale Data Poisoning: Split-View Attack matter for security teams?
• Why does Layer 2 VPN matter for security teams?