CVE-2023-36673: 
Cisco AnyConnect Secure Client vulnerability analysis and mitigation

An issue was discovered in Avira Phantom VPN through 2.23.1 for macOS where the VPN client insecurely configures the operating system such that all IP traffic to the VPN server's IP address is sent in plaintext outside the VPN tunnel, even if this traffic is not generated by the VPN client, while simultaneously using plaintext DNS to look up the VPN server's IP address. This vulnerability, identified as CVE-2023-36673, allows an adversary to trick the victim into sending traffic to arbitrary IP addresses in plaintext outside the VPN tunnel (NVD).

The vulnerability is part of a broader set of VPN vulnerabilities known as TunnelCrack. This specific issue, known as a ServerIP attack, involves DNS spoofing combined with routing table manipulation. The attack has a CVSS v3.1 base score of 7.3 (HIGH) with a vector string of CVSS:3.1/AV:A/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N. The vulnerability is classified under CWE-319 (Cleartext Transmission of Sensitive Information) (NVD, TunnelCrack).

The vulnerability allows attackers to read and intercept transmitted traffic, potentially exposing sensitive user information. Even if the victim is using another layer of encryption such as HTTPS, the attacks can reveal which websites a user is visiting, posing a significant privacy risk. The attack can be executed by untrusted Wi-Fi networks and malicious internet service providers (TunnelCrack, Help Net Security).

For customers using Prisma Access, they are not impacted by ServerIP attacks. For PAN-OS with GlobalProtect Gateways configured with the address set as an IP rather than FQDN, they are also not impacted. Additionally, endpoints configured to use only an authenticated DNS mechanism such as DNS over TLS, DNS over HTTPS, or DNSSEC are protected from the ServerIP attacks (Palo Alto).

Several major VPN vendors have responded to the disclosure by releasing patches. Some example patched VPNs include Mozilla VPN, Surfshark, Malwarebytes, Windscribe, and Cloudflare's WARP. The vulnerability was discovered by Dr. Mathy Vanhoef and team, who coordinated with CERT/CC and various VPN vendors during a 90-day disclosure period (TunnelCrack).