Why Your VPN Might Be Leaking Your Location and Credentials

TLDR: Network security monitoring at large-scale events like Black Hat reveals that many users rely on misconfigured or low-quality VPNs that leak sensitive data in the clear. Even when a VPN tunnel appears active, traffic often bypasses encryption for location data and authentication credentials, exposing users to trivial interception. Pentesters should prioritize testing VPN client behavior under network-restricted conditions to identify these common leaks.

Security professionals often treat a VPN as a silver bullet for privacy, especially when connecting to untrusted networks at conferences or coffee shops. The reality, as observed in the network operations center at Black Hat Asia, is far more precarious. When you flip that switch to "on," you are trusting the client software to correctly route all traffic through an encrypted tunnel. Our monitoring consistently shows that this trust is frequently misplaced. We observed multiple instances where VPN clients leaked precise GPS coordinates and authentication credentials in the clear, despite the user interface indicating a secure connection.

The Illusion of the Secure Tunnel

The core issue stems from how different VPN implementations handle traffic routing and DNS resolution. When a client fails to properly enforce a "kill switch" or mismanages routing tables, traffic can leak outside the encrypted tunnel. This is not just a theoretical risk; it is a persistent misconfiguration that we see across various providers.

In one specific case, we identified a VPN client that successfully encrypted the primary data stream but failed to secure the metadata associated with the device's location. By monitoring the unencrypted traffic, we could map the exact physical location of the user within the conference center using Google Maps. For a researcher or a high-value target, this level of exposure is catastrophic. If an attacker can correlate your physical location with your network traffic, the anonymity provided by the VPN is effectively nullified.

Credentials in the Clear

Beyond location data, we frequently see authentication traffic leaking over unencrypted channels. This often happens during the initial handshake or when the client attempts to re-authenticate after a network drop. We have observed clear-text SMTP and HTTP traffic containing sensitive credentials, which provides an easy win for anyone performing active scanning or simple packet sniffing on the local segment.

For those of you performing penetration tests, this is a goldmine. When you are on an engagement, do not assume the client’s VPN is doing its job. Use tools like Zeek or NetWitness to inspect the traffic flowing from the client machine. You will often find that while the bulk of the traffic is encrypted, specific application-layer protocols or system-level requests are being sent in the clear.

If you are testing a mobile application or a desktop client, look for these patterns:

# Example of monitoring for clear-text traffic on a specific interface
tcpdump -i eth0 -A 'tcp port 80 or port 25'

This simple command can often reveal whether an application is failing to enforce TLS or if the VPN client is leaking specific packets. The impact of these leaks is severe. An attacker can perform injection attacks or session hijacking by simply sitting on the same network and waiting for the client to misstep.

The Log4j Legacy and Beyond

The prevalence of these leaks is exacerbated by the fact that many users are running outdated or vulnerable software. We still see significant traffic attempting to exploit CVE-2021-44228, commonly known as Log4j. When a user connects to a network with a vulnerable machine, they are not just exposing their own data; they are providing a foothold for lateral movement.

The combination of a leaking VPN and a vulnerable application is a recipe for disaster. We have seen instances where an attacker used the information leaked by a VPN to identify the specific version of an application running on the client, then immediately followed up with a targeted exploit. This is not the work of sophisticated nation-state actors; it is the work of opportunistic individuals who understand that most users have no idea what is actually leaving their machine.

Defensive Realities

Defenders need to move beyond the assumption that a VPN provides a "robust" perimeter. You must implement egress filtering and deep packet inspection to identify when traffic is bypassing your security controls. If your organization relies on VPNs for remote access, you should be auditing the client configurations to ensure that split-tunneling is disabled and that all DNS queries are forced through an encrypted resolver.

If you are a researcher, the next time you are at a conference, take a moment to look at what your own machine is doing. You might be surprised to find that your "secure" connection is broadcasting your location and credentials to anyone with a passive listener. Trust is a vulnerability. Verify your traffic, and do not assume that a green icon on your screen means your data is safe.