Common mistakes with VPN Split Tunneling are sending sensitive traffic outside the tunnel, allowing insecure local network access, and failing to separate trusted and untrusted destinations. Misconfiguration can expose internal resources, DNS queries or credentials to the local network or the internet, so teams should scope exclusions tightly and test routing behaviour.
How Split Tunneling Creates Exposure When It Is Scoped Poorly
Split tunneling is not inherently unsafe, but it becomes risky when the policy boundary is vague. The common failure is letting the VPN protect only some traffic while other traffic, especially DNS, cloud apps, file shares, or admin tools, still leaves the device on an untrusted path. That breaks the assumption that the endpoint has a single protected route.
A second problem is that split tunneling can make local network access behave differently from remote access in ways users do not notice. If the laptop can still talk to printers, peer devices, or captive portals while also reaching internal systems, attackers on the same local network may get a chance to observe, redirect, or influence traffic that teams assumed was protected.
There is also a trust-boundary problem: teams often exclude destinations because they are “safe” or “low risk” without defining that boundary tightly enough. A split-tunnel rule set should be treated as a routing policy, not just a convenience setting, because every exclusion expands the set of traffic that depends on the security of the local network, the ISP, or the destination service.
Where Split Tunneling Usually Goes Wrong in Practice
The most common mistake is excluding too much. That can send sensitive web traffic, internal application calls, DNS lookups, or authentication flows outside the tunnel, where they are exposed to interception or manipulation on an untrusted network. It also creates inconsistent user experience, because some apps appear protected while others silently bypass the VPN.
Another frequent error is leaving insecure local network access enabled by default. When split tunneling is combined with broad local LAN reachability, the device may remain reachable by nearby hosts while still holding corporate access. That is especially dangerous on hotel, conference, home, or shared networks, where the local segment cannot be assumed trustworthy.
A third mistake is failing to distinguish between destinations that are truly public and those that merely look public. Many SaaS services, identity endpoints, update services, and content delivery paths can still be security-sensitive because they carry session cookies, tokens, or device trust signals. VPN Security Guide covers the broader remote-access design choices that help teams decide what should stay inside the protected path.
Routing, DNS, and Trust Boundaries Need Separate Testing
Split tunneling should be validated at the packet and name-resolution level, not just by checking whether the VPN shows “connected.” DNS is a frequent blind spot because the application traffic may be tunneled while the lookup itself still leaks to the local resolver. That can reveal internal hostnames, service names, or destinations even when the payload is protected.
Teams also need to test what happens when the VPN drops, reconnects, or changes routes. If traffic fails open, a user may continue working over the local network without noticing. If traffic fails closed too aggressively, users may bypass the control altogether. The right answer is not simply “all or nothing”, but a clearly tested routing model with predictable failover behaviour.
Session handling matters as well. If a protected session can be resumed from an unprotected path, or if a device can reach an internal resource through one interface while using credentials learned on another, the trust boundary is weaker than it looks. This is why remote-access policy should be checked alongside session policy, not as a separate afterthought.
Risk and Threat Considerations
Split tunneling widens the attack surface because it creates two trust zones on the same endpoint. Sensitive traffic may leave the tunnel, local attackers may gain a view of device activity, and exposed DNS or authentication traffic can reveal internal targets or enable interception on hostile networks.
Failure mechanism: Mis-scoped exclusions, permissive local network access, or incomplete DNS handling allow traffic that should be protected to traverse an untrusted path, or allow an attacker on the local segment to observe and influence communications.
Impact: The organisation can leak internal names, session material, or credential-related traffic, and in the worst case expose reachable internal services to reconnaissance, hijacking, or follow-on compromise. NIST Cybersecurity Framework 2.0 is useful here because it frames the control problem as a combination of protect, detect, and recover discipline rather than a single VPN setting.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
NIST Zero Trust (SP 800-207), NIST SP 800-53 Rev 5 and CIS Controls v8 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| NIST Zero Trust (SP 800-207) | PR.AA-05 — Least Privilege for Identities and Accesses | Split tunneling changes trust boundaries and access paths, so least privilege directly shapes which traffic may bypass the tunnel. |
| Recommendation — Limit split-tunnel exclusions to the minimum traffic needed and keep sensitive access inside the protected path. | ||
| NIST SP 800-53 Rev 5 | SC-7 — Boundary Protection | Split tunneling is fundamentally a boundary-control issue because it separates protected from unprotected network flows. |
| IA-5 — Authenticator Management | Leaked DNS, credential, or session traffic makes authenticator handling part of the split-tunnel risk. | |
| Recommendation — Enforce boundary controls that define, inspect, and restrict traffic allowed to leave the VPN tunnel. Protect and rotate authenticators that could be exposed by traffic leaving the tunnel. | ||
| CIS Controls v8 | CIS-12 — Network Infrastructure Management | VPN split tunneling is a network routing and segmentation decision that needs controlled configuration and validation. |
| CIS-6 — Access Control Management | Exclusions and local-network reachability are access decisions that can widen exposure if not tightly managed. | |
| Recommendation — Review VPN routing, DNS, and local network settings as part of network configuration management. Restrict split-tunnel exceptions to approved use cases and revalidate them regularly. | ||
Practitioner Guidance
What to verify: Confirm which destinations are excluded, how DNS is handled, whether local LAN access is permitted, and what happens during VPN disconnects. Test from an untrusted network, not only from the office, because split tunneling failures often appear only when the device is off-premises.
Decision rule: If a destination can carry sensitive data, identity traffic, or internal admin access, keep it in the tunnel unless there is a documented business reason to exclude it and a compensating control to match. If the app is public but the session or token is sensitive, treat it as protected traffic.
Common mistake: Treating split tunneling as a performance optimisation only. The real decision is where trust is placed, and every exclusion should be reviewed like an access exception, not a convenience setting.
Practitioner takeaway: The goal is not to eliminate split tunneling, but to make every bypass explicit, narrow, and testable so that “convenience” does not quietly become a data-leak path.