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Virtual Private Networks (VPNs) have long been championed as the go-to solution for safeguarding internet privacy, particularly when navigating untrusted networks like public Wi-Fi. However, new research reveals a surprising vulnerability that could allow attackers on the same network to bypass the protection provided by VPNs without triggering any alerts to the user. Let’s explore this hidden weakness and the implications it holds for internet security.

The Mechanics of a VPN Connection

When you connect to a network, your device broadcasts a request for an IP address. Typically, the network’s Dynamic Host Configuration Protocol (DHCP) server assigns this address and sets the gateway for internet access. VPNs create a secure, encrypted tunnel for your internet traffic through a virtual network interface, shielding your data from prying eyes.

However, researchers at Leviathan Security have discovered a flaw within the DHCP standard that can be exploited to reroute traffic away from this secure tunnel. This exploitation, specifically of DHCP option 121, allows attackers to set their own routing rules that take precedence over those of the VPN.

The Exploitation Process

Here’s how it works: An attacker sets up a rogue DHCP server on the same network as the target. This rogue server can then issue a gateway directive, effectively rerouting the target’s traffic through the attacker’s machine instead of the VPN’s encrypted tunnel. The attacker can then snoop on this traffic, gaining access to potentially sensitive information.

This method relies on abusing DHCP option 121, which allows for more specific routing rules than those typically set by VPNs. By leveraging this option, attackers can ensure that certain traffic bypasses the VPN entirely, without the user ever being aware.

Practical Implications

While this attack won’t grant access to all of a target’s internet traffic—since much of it is encrypted (indicated by “https://” in URLs)—attackers can still harvest metadata. This includes source and destination addresses, which can be valuable in spear-phishing or other targeted attacks.

Bill Woodcock, executive director at Packet Clearing House, emphasizes the potential danger for high-value targets. Individuals in positions of authority, or those with significant wealth, could be particularly at risk. Attackers could easily exploit this vulnerability in places where these individuals frequent, like coffee shops or cafes, making the attack highly effective and undetectable.

Defense Strategies

Leviathan Security recommends several strategies to mitigate this risk:

  1. Use Android Devices: Android devices reportedly ignore DHCP option 121, rendering this attack ineffective.
  2. Personal Hotspots: Employing a personal cellular hotspot can create a private, password-protected network, cutting off potential attackers.
  3. Virtual Machines: Running your VPN inside a virtual machine (VM) can thwart this attack, provided the VM is not in bridged mode.
  4. Deep Packet Inspection: This can block all non-essential traffic, although it introduces potential side-channel vulnerabilities.

A Call for Realistic Expectations

Lizzie Moratti from Leviathan Security underscores a crucial point: VPNs are designed to protect your data over the internet, not necessarily on local networks. VPN providers often promise comprehensive security that their technology can’t fully deliver, especially against sophisticated local network attacks.


This research serves as a stark reminder that no security solution is foolproof. Users must remain vigilant and adopt additional protective measures when using VPNs on untrusted networks. By understanding and mitigating these vulnerabilities, we can better protect our digital privacy in an increasingly interconnected world.

For more detailed information, including the research findings and replication code, Leviathan Security has made their full study available here.



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