VPN SSL Interception: Risks & Checks

TL;DR

A VPN provider could intercept your SSL traffic (Man-in-the-Middle attack), but it’s rare and detectable. This guide explains how, why, and what you can do to protect yourself.

Understanding the Risk

When you connect to a website using HTTPS (SSL/TLS), your computer verifies the website’s identity with a digital certificate. A VPN sits between your device and the internet. If a VPN provider replaces the legitimate website’s certificate with its own, it can decrypt your traffic, inspect it, and re-encrypt it before sending it on. This is SSL interception.

Why would a VPN do this?

• Malicious Intent: The most concerning reason – data theft, tracking, or injecting malware.
• Certificate Pinning Issues: Some apps use certificate pinning, which can cause connection problems if the VPN’s certificate isn’t trusted. A poorly implemented workaround might involve interception.
• Traffic Inspection (Rare): Some VPNs advertise features like ad-blocking or malware scanning that could require inspecting traffic, but this is usually done without full SSL decryption.

How to Detect SSL Interception

1. Check the VPN Provider’s Reputation: Research the provider thoroughly before subscribing. Look for independent audits and reviews.
2. Use a Certificate Checker Tool: These tools show you the certificate details your browser is using when connected to the VPN.
   • Online Tools: Websites like SSL Shopper or DigiCert SSL Analyzer can help.
3. Examine the Certificate Chain: When connected to a VPN, verify that the certificate chain leads back to a trusted root authority and doesn’t include unexpected intermediate certificates issued by the VPN provider.
   • Browser Instructions (Chrome): Click the padlock icon in your address bar > Connection is secure > Certificate is valid. Expand ‘Certificate Path’ to view the chain.
   • Browser Instructions (Firefox): Click the padlock icon > More Information > View Certificate. Go to the ‘Details’ tab and examine the certificate hierarchy.
4. Use a DNS Leak Test: While not directly related to SSL interception, it confirms your VPN is routing traffic correctly.
   • Online Tools: DNSLeakTest is a good option.
5. Check for Unexpected Redirects: If you’re redirected to different websites than expected, it could indicate manipulation.
6. Look for Browser Warnings: Pay attention to any security warnings your browser displays about invalid certificates or untrusted connections.

Technical Checks (Advanced)

These require more technical knowledge.

1. Wireshark Analysis: Use Wireshark to capture network traffic and inspect the SSL handshake process.
   • Install Wireshark: Download Wireshark
   • Capture Traffic: Start a capture on your VPN interface.
   • Filter for TLS Handshake: Use the filter tls.handshake to focus on SSL/TLS negotiations.
   • Examine Certificates: Look for certificates issued by the VPN provider instead of the expected website.
2. OpenSSL Command Line (Example): You can use OpenSSL to verify certificate details directly.

   openssl s_client -connect example.com:443

   Examine the output for the certificate chain and issuer information.

Protecting Yourself

• Choose a Reputable VPN Provider: Prioritize providers with strong privacy policies, independent audits, and transparent logging practices.
• Enable Perfect Forward Secrecy (PFS): PFS ensures that past communication sessions cannot be decrypted even if the private key is compromised. Most good VPNs support this by default.
• Use a Strong Encryption Protocol: WireGuard or OpenVPN are generally considered more secure than older protocols like PPTP or L2TP/IPsec.
• Keep Your Software Updated: Regularly update your operating system, browser, and VPN client to patch security vulnerabilities.