Researchers have detailed a Digital Non-public Community (VPN) bypass approach dubbed TunnelVision that permits menace actors to listen in on sufferer’s community visitors by simply being on the identical native community.
The “decloaking” methodology has been assigned the CVE identifier CVE-2024-3661 (CVSS rating: 7.6). It impacts all working techniques that implement a DHCP shopper and has assist for DHCP possibility 121 routes.
At its core, TunnelVision entails the routing of visitors with out encryption via a VPN by the use of an attacker-configured DHCP server utilizing the classless static route possibility 121 to set a route on the VPN consumer’s routing desk.
It additionally stems from the very fact the DHCP protocol, by design, doesn’t authenticate such possibility messages, thus exposing them to manipulation.
DHCP is a shopper/server protocol that mechanically supplies an Web Protocol (IP) host with its IP handle and different associated configuration data such because the subnet masks and default gateway in order to entry the community and its sources.
It additionally helps reliably configure IP addresses by way of a server that maintains a pool of IP addresses and leases an handle to any DHCP-enabled shopper when it begins up on the community.
As a result of these IP addresses are dynamic (i.e., leased) moderately than static (i.e., completely assigned), addresses which are not in use are mechanically returned to the pool for reallocation.
The vulnerability, in a nutshell, makes it attainable for an attacker with the flexibility to ship DHCP messages to govern routes to redirect VPN visitors, thereby permitting them to learn, disrupt, or probably modify community visitors that was anticipated to be protected by the VPN.
“As a result of this method just isn’t depending on exploiting VPN applied sciences or underlying protocols, it really works fully independently of the VPN supplier or implementation,” Leviathan Safety Group researchers Dani Cronce and Lizzie Moratti stated.
“Our approach is to run a DHCP server on the identical community as a focused VPN consumer and to additionally set our DHCP configuration to make use of itself as a gateway. When the visitors hits our gateway, we use visitors forwarding guidelines on the DHCP server to move visitors via to a professional gateway whereas we listen in on it.”
In different phrases, TunnelVision tips a VPN consumer into believing that their connections are secured and routed via an encrypted tunnel, when in actuality it has been redirected to the attacker’s server in order that it may be doubtlessly inspected.
Nonetheless, to be able to efficiently decloak the VPN visitors, the focused host’s DHCP shopper should implement DHCP possibility 121 and settle for a DHCP lease from the attacker-controlled server.
The assault can also be much like TunnelCrack, which is designed to leak visitors exterior a protected VPN tunnel when connecting to an untrusted Wi-Fi community or a rogue ISP, leading to adversary-in-the-middle (AitM) assaults.
The issue impacts all main working techniques like Home windows, Linux, macOS, and iOS except Android because it doesn’t have assist for DHCP possibility 121. It additionally impacts VPN instruments that solely depend on routing guidelines to safe the host’s visitors.
Mullvad has since confirmed that the desktop variations of its software program have firewall guidelines in place to dam any visitors to public IPs exterior the VPN tunnel, however acknowledged that the iOS model is weak to TunnelVision.
Nonetheless, it is but to combine and ship a repair owing to the complexity of the enterprise, which the Swedish firm stated has been engaged on for “a while.”
“The TunnelVision vulnerability (CVE-2024-3661) exposes a technique for attackers to bypass VPN encapsulation and redirect visitors exterior the VPN tunnel,” Zscaler researchers stated, describing it as a way that employs a DHCP hunger assault to create a side-channel.
“This system entails utilizing DHCP possibility 121 to route visitors with out encryption via a VPN, finally sending it to the web by way of a side-channel created by the attacker.”
To mitigate TunnelVision, organizations are beneficial to implement DHCP snooping, ARP protections, and port safety on switches. It is also suggested to implement community namespaces on Linux to repair the conduct.