A team of researchers from universities in London and Rome have published a paper in which they claim that many of the world's top VPN providers leak IPv6 traffic. TorrentFreak has spoken to several companies highlighted in the report and today we publish their responses.

As Internet users seek to bypass censorship, boost privacy and achieve a level of anonymity, VPN services have stepped in with commercial solutions to assist with these aims. The uptake among consumers has been impressive.

Reviews of VPN services are commonplace and usually base their ratings on price and speed. At TorrentFreak we examine many services annually, but with a focus on privacy issues instead.

Now a team of researchers from universities in London and Rome have published a paper titled A Glance through the VPN Looking Glass: IPv6 Leakage and DNS Hijacking in Commercial VPN clients. (pdf) after investigating 14 popular services on the market today.

“Our findings confirm the criticality of the current situation: many of these providers leak all, or a critical part of the user traffic in mildly adversarial environments. The reasons for these failings are diverse, not least the poorly defined, poorly explored nature of VPN usage, requirements and threat models,” the researchers write.

While noting that all providers are able to successfully send data through an encrypted tunnel, the paper claims that problems arise during the second stage of the VPN client’s operation: traffic redirection.

“The problem stems from the fact that routing tables are a resource that is concurrently managed by the operating system, which is unaware of the security requirements of the VPN client,” the researchers write.

This means that changes to the routing table (whether they are malicious or accidental) could result in traffic circumventing the VPN tunnel and leaking to other interfaces.

IPv6 VPN Traffic Leakage

“The vulnerability is driven by the fact that, whereas all VPN clients manipulate the IPv4 routing table, they tend to ignore the IPv6 routing table. No rules are added to redirect IPv6 traffic into the tunnel. This can result in all IPv6 traffic bypassing the VPN’s virtual interface,” the researchers explain.

As illustrated by the chart above, the paper claims that all desktop clients (except for those provided by Private Internet Access, Mullvad and VyprVPN) leaked “the entirety” of IPv6 traffic, while all providers except Astrill were vulnerable to IPv6 DNS hijacking attacks.

The paper was covered yesterday by The Register with the scary-sounding title “VPNs are so insecure you might as well wear a KICK ME sign” but without any input from the providers in question. We decided to contact a few of them for their take on the paper.

PureVPN told TF that they “take the security of our customers very seriously and thus, a dedicated team has been assigned to look into the matter.” Other providers had already received advanced notice of the paper.

“At least for AirVPN the paper is outdated,” AirVPN told TorrentFreak.

“We think that the researchers, who kindly sent the paper to us many months in advance and were warned about that, had no time to fix [the paper] before publication. There is nothing to worry about for AirVPN.”

“Current topology allows us to have the same IP address for VPN DNS server and VPN gateway, solving the vulnerability at its roots, months before the publication of the paper.”

TorGuard also knew of the whitepaper and have been working to address the issues it raises. The company adds that while The Register’s “the sky is falling” coverage of yesterday is “deceptive”, the study does illustrate the need for providers to stay vigilant. Specifically, TorGuard says that it has launched a new IPv6 leak prevention feature on Windows, Mac and Linux.

“Today we have released a new feature that will address this issue by giving users the option of capturing ALL IPv6 traffic and forcing it through the OpenVPN tunnel. During our testing this method proved highly effective in blocking potential IPv6 leaks, even in circumstances when these services were active or in use on the client’s machine,” the company reports.

On the DNS hijacking issue, TorGuard provides the following detail.

“It is important to note that the potential for this exploit only exists (in theory) if you are connected to a compromised WiFi network in which the attacker has gained full control of the router. If that is the case, DNS hijacking is only the beginning of one’s worries,” TorGuard notes.

“During our own testing of TorGuard’s OpenVPN app, we were unable to reproduce this when using private DNS servers because any DNS queries can only be accessed from within the tunnel itself.”

Noting that they released IPv6 Leak Protection in October 2013, leading VPN provider Private Internet Access told TorrentFreak that they feel the paper is lacking.

“While the article purported to be an unbiased and intricate look into the security offered by consumer VPN services, it was greatly flawed since the inputs or observations made by the researchers were inaccurate,” PIA said.

“While a scientific theory or scientific test can be proven by a logical formula or algorithm, if the observed or collected data is incorrect, the conclusion will be in error as well.”

PIA criticizes the report on a number of fronts, including incorrect claims about its DNS resolver.

“Contrary to the report, we have our own private DNS daemon running on the Choopa network. Additionally, the DNS server that is reported, while it is a real DNS resolver, is not the actual DNS that your system will use when connected to the VPN,” the company explains.

“Your DNS requests are handled by a local DNS resolver running on the VPN gateway you are connected to. This can be easily verified through a site like ipleak.net. Additionally… we do not allow our DNS servers to report IPv6 (AAAA records) results. We’re very serious about security and privacy.”

Finally, in a comprehensive response (now published here) in which it notes that its Windows client is safe, PIA commends the researchers for documenting the DNS hijacking method but criticizes how it was presented to the VPN community.

“The DNS Hijacking that the author describes [..] is something that has recently been brought to light by these researchers and we commend them on their discovery. Proper reporting routines would have been great, however. Shamefully, this is improper security disclosure,” PIA adds.

While non-IPv6 users have nothing to fear, all users looking for a simple fix can disable IPv6 by following instructions for Windows, Linux and Mac.