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Publish Date: 5/6/2024

How was this found?

We researched VPN threat models for a project and produced methods to attack VPNs. The team’s idea proved to be a severe issue, and we suspected it affected the entirety of the VPN industry. That’s when we began research in earnest.

Has this been exploited in the wild?

We have not seen evidence as of 5/6/2024 of exploitation in the wild. However, we believe this was possible as far back as 2002, so it’s possible someone besides us found TunnelVision but didn’t widely disclose or realize the severity.

In order to detect this, you would need to be reviewing DHCP network traffic and DHCP client logs. This might be possible retroactively.

Haven’t routing table leaks in VPNs been known for years?

Correct, routing table issues have been known for years. This is why the privacy and anonymity communities discourage VPNs. Most issues found have been small and patchable leaks.

How is this novel?

We have not seen any prior research using DHCP option 121 to inject routes and decloak VPNs, which is the novel part of our research. It also is not completely patchable in the same way smaller leaks have been (besides on Linux). We pointed to prior research on this topic related to routing table issues that have been known for years that we were able to find.

UPDATE: The purpose of this research was to test this technique against modern VPN providers to determine their vulnerability and to notify the wider public of this issue. This is why we agreed with CISA to file a CVE when we disclosed to them and why we decided to name the vulnerability.

After publication, we have received details about prior research into combining routing table behavior with option 121. These researchers showed that at least some people were aware of DHCP option 121’s effect on VPNs going back to at least 2015. Despite this, the research did not lead to wide deployment of mitigations nor the general public awareness of the decloaking behavior.

That said, we are grateful for the input of past researchers who explored the problem space. We will continue to credit our fellow security researchers who explored these issues.

Prior works referencing DHCP 121 route injection or VPN decloaking:

Wouldn’t exploiting this be obvious?

Not really. The VPN user shows as being connected to the VPN. Attackers can control which IPs they wish to decloak, so theoretically they could choose to not decloak leak checking IPs. They could also leak DNS IPs while forwarding the traffic so the VPN connection is intact, but they obtain information about the traffic in the tunnel.

In no circumstances did we observe a VPN server disconnecting us with kill switches or other features.

Aren’t you going to be snooping on encrypted packets?

The packets will not be encrypted. DHCP option 121 will alter your routing table and send your traffic over a non-VPN interface, completely bypassing the encryption routine VPN application processes normally do. You can’t encrypt what you never receive.

Isn’t most web traffic encrypted with HTTPS anyways?

This is true. If HTTPS traffic is decloaked it is still not possible to view the encrypted contents of the packet. For unencrypted protocols, the packet’s payload is readable.

However, it is still possible to see the destination and the protocol of the packet. Normally, that information would be inside the VPN protocol’s payload and encrypted.

Why are you saying the mitigation’s side-channel is so impactful?

When a mitigation is in place, an attacker can still obtain the destination you are talking to and the amount of traffic you are sending to that destination. Since most web traffic is encrypted with HTTPS, attackers can usually only determine the destination and protocol of the traffic anyway. This means the vulnerability that the mitigation introduces has nearly the same impact as decloaking.

The side-channel is also flexible in use:

  1. The traffic can be checked against a predefined list of IP addresses.
  2. The traffic can be selectively denied as a censorship mechanism.
  3. The attacker can use IP space denial with binary search to determine all current connections in logarithmic time.

Why is this so bad compared to other leaks?

VPN providers often claim that no one, not even a network administrator, is able to remove your VPN protections. They may state it as “securing you on an untrusted network.”

TunnelVision shows that a network administrator can completely remove VPN protections. To make matters worse, we’ve demonstrated that an attacker who is on the same network has ways to control DHCP. This is demonstrated in our POC video

Don’t most networks have widespread adoption of DHCP snooping or other protections?

Most enterprise networks do. However, most people using VPNs are not connecting from an enterprise network. They are connecting from public networks or their home networks.

In addition, part of the VPN provider threat model is they can secure any untrusted network, including those who do not have these protections.

Can you change the configuration of the DHCP server in the middle of a VPN session?

Yes. By setting a short DHCP lease length, an attacker can modify the configurations in the middle of a VPN session.