Split Tunnel VPN Security Risks

TL;DR

Split tunnel VPNs can leak your traffic outside the secure connection if not configured correctly. This guide explains how attackers exploit this and what you can do to protect yourself.

Understanding Split Tunneling

A split tunnel VPN sends some of your internet traffic through a Virtual Private Network (VPN) while other traffic goes directly to the internet, bypassing the VPN. This is often done for performance reasons or to access local network resources. The risk comes when you assume all your traffic is protected by the VPN.

Attack Scenario: DNS Leaks

An attacker can exploit split tunneling if your Domain Name System (DNS) requests aren’t routed through the VPN. Here’s how:

1. Traffic Interception: You connect to a website (e.g., example.com).
2. DNS Request Leak: Your computer sends a DNS request to your default DNS server, which might be your Internet Service Provider’s (ISP) server instead of the VPN’s.
3. Attacker Monitoring: The attacker, positioned on the network or controlling the ISP’s DNS server, sees your DNS requests and can log the websites you are visiting.
4. Correlation: Even if the website traffic itself is encrypted (HTTPS), knowing which domains you request reveals a lot about your activity.

Attack Scenario: IP Address Exposure

If application traffic isn’t routed through the VPN, your real IP address can be exposed.

1. Application Bypass: Some applications (e.g., certain games, peer-to-peer software) might bypass the VPN connection entirely.
2. Direct Connection: These applications connect directly to the internet using your public IP address.
3. Attacker Identification: An attacker can see your real IP address and location, potentially linking it back to you.

How to Protect Yourself

Here’s how to mitigate split tunnel VPN security risks:

1. Force All Traffic Through the VPN

This is the most secure option, but may impact performance.

1. VPN Client Settings: Most VPN clients have an option called “Force all traffic through VPN” or similar. Enable this setting.
2. Operating System Configuration (Advanced): You can configure your operating system’s routing table to ensure all traffic goes through the VPN interface. This is more complex and requires technical knowledge. For example, on Linux:

   sudo ip route add default via [VPN_GATEWAY_IP] dev [VPN_INTERFACE]

3. Firewall Rules: Configure your firewall to block all outbound traffic except through the VPN interface.

2. Verify DNS Leak Protection

Confirm that your DNS requests are going through the VPN.

1. DNS Leak Test Websites: Use websites like DNSLeakTest to check for leaks.
2. VPN Provider’s DNS Servers: Ensure your VPN provider provides dedicated DNS servers and that you are using them. Check your network adapter settings or VPN client configuration.

3. Application-Level Control

If you need to allow specific applications to bypass the VPN, be cautious.

1. Split Tunnel Configuration: Carefully configure split tunneling rules in your VPN client. Only allow trusted applications to bypass the VPN.
2. Monitor Application Traffic: Use network monitoring tools (e.g., Wireshark) to verify that only allowed applications are bypassing the VPN.

4. Kill Switch

A kill switch automatically disconnects your internet connection if the VPN connection drops, preventing traffic from leaking.

1. VPN Client Feature: Most reputable VPN clients include a kill switch feature. Enable it in the settings.

5. Regular Security Audits

Periodically review your VPN configuration and security practices to ensure they are still effective.