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On Wednesday, security researchers from the University of New Mexico disclosed a vulnerability impacting most Linux distributions and Unix-like operating systems including FreeBSD, OpenBSD, macOS, iOS, and Android. This Linux vulnerability can be exploited by an attacker to determine if a user is connected to a VPN and to hijack VPN connections.

The researchers shared that this security flaw tracked as CVE-2019-14899, “allows a network adjacent attacker to determine if another user is connected to a VPN, the virtual IP address they have been assigned by the VPN server, and whether or not there is an active connection to a given website.” Additionally, attackers can determine the exact sequence and acknowledgment numbers by counting encrypted packets or by examining their size. With this information in hand, they can inject arbitrary data payloads into IPv4 and IPv6 TCP streams.

What systems are affected by this Linux vulnerability

While testing for this vulnerability, the researchers found that it did not affect any Linux distribution prior to Ubuntu 19.10. They further noted that all distributions that use ‘systemd’ versions released after November 28, 2018, that have their rp_filter (reverse path filtering) set to “loose” by default are vulnerable.

Here’s a non-exhaustive list of systems that the researchers found vulnerable:

  • Ubuntu 19.10 (systemd)
  • Fedora (systemd)
  • Debian 10.2 (systemd)
  • Arch 2019.05 (systemd)
  • Manjaro 18.1.1 (systemd)
  • Devuan (sysV init)
  • MX Linux 19 (Mepis+antiX)
  • Void Linux (runit)
  • Slackware 14.2 (rc.d)
  • Deepin (rc.d)
  • FreeBSD (rc.d)
  • OpenBSD (rc.d)

Attacks exploiting this Linux vulnerability works against OpenVPN, WireGuard, and IKEv2/IPSec. However, the team noted they were able to make all the inferences even when the responses from the victim were encrypted. Regardless of what VPN technology you are using, the size and number of packets sent were enough to find the kind of packets are being sent through the encrypted VPN tunnel.

In response to the public disclosure, Jason A. Donenfeld, the creator of the WireGuard, clarified that “this isn’t a WireGuard vulnerability, but rather something in the routing table code and/or TCP code on affected operating systems.” He added, “However, it does affect us, since WireGuard exists on those affected OSes.” A network security consultant Noel Kuntze also said in a reply to the disclosure report that only route-based VPN implementations are impacted by this Linux vulnerability.

The researchers have also shared a few mitigation strategies including turning reverse path filtering on, using bogon filtering, and encrypting packet size and timing. You can check out the full disclosure report of this Linux vulnerability for further details.

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