6.5 - VU#199397

Overview

Tunnelling protocols are an essential part of the Internet and form much of the backbone that modern network infrastructure relies on today. One limitation of these protocols is that they do not authenticate and/or encrypt traffic. Though this limitation exists, IPsec can be implemented to help prevent attacks. However, implementation of these protocols have been executed poorly in some areas.

For the latest security findings from the researchers at the DistriNet-KU Leuven research group, please refer to: https://papers.mathyvanhoef.com/usenix2025-tunnels.pdf

Description

Researchers at the DistriNet-KU Leuven research group have discovered millions of vulnerable Internet systems that accept unauthenticated IPIP, GRE, 4in6, or 6in4 traffic. This can be considered a generalization of the vulnerability in VU#636397 : IP-in-IP protocol routes arbitrary traffic by default (CVE-2020-10136). The exposed systems can be abused as one-way proxies, enable an adversary to spoof the source address of packets (CWE-290 Authentication Bypass by Spoofing), or permit access to an organization's private network. Vulnerable systems can also facilitate Denial-of-Service (DoS) attacks. Two types of DoS attacks exploiting this vulnerability can amplify traffic: one concentrates traffic in time ("Tunneled-Temporal Lensing"), and the other can loop packets between vulnerable systems, resulting in an amplification factor of at least 13- and 75-fold, respectively. Additionally, the researchers discovered an Economic Denial of Sustainability (EDoS), where the outgoing bandwidth of a vulnerable system is drained, raising the cost of operations if hosted by a third-party cloud service provider.

Impact

An adversary can abuse these security vulnerabilities to create one-way proxies and spoof source IPv4/6 addresses. Vulnerable systems may also allow access to an organization's private network or be abused to perform DDoS attacks.

Solution

See the "Defences" section in the researcher's publication https://papers.mathyvanhoef.com/usenix2025-tunnels.pdf

Acknowledgements

Thanks to the researchers Mathy Vanhoef and Angelos Beitis of the DistriNet-KU Leuven research group for the initial discovery and research. This document was written by Ben Koo.

CVE-2024-7595 GRE and GRE6 Protocols (RFC2784) do not validate or verify the source of a network packet, allowing an attacker to route arbitrary traffic via an exposed network interface that can lead to spoofing, access control bypass, and other unexpected network behaviors. This can be considered similar to CVE-2020-10136.

CVE-2024-7596 Proposed Generic UDP Encapsulation (GUE) (IETF draft-ietf-intarea-gue*) does not validate or verify the source of a network packet, allowing an attacker to route arbitrary traffic via an exposed network interface that can lead to spoofing, access control bypass, and other unexpected network behaviors. This can be considered similar to CVE-2020-10136.

*Note: GUE Draft is expired and no longer canonical.

CVE-2025-23018 The IPv4-in-IPv6 and IPv6-in-IPv6 protocols (RFC2473) do not require the validation or verification of the source of a network packet, allowing an attacker to route arbitrary traffic via an exposed network interface that can lead to spoofing, access control bypass, and other unexpected network behaviors. This can be considered similar to CVE-2020-10136.

CVE-2025-23019 The IPv6-in-IPv4 protocol (RFC4213) does not require authentication of incoming packets, allowing an attacker to route traffic via an exposed network interface that can lead to spoofing, access control bypass, and other unexpected network behaviors.

Note: CVE-2024-7595, CVE-2024-7596, and CVE-2025-23018 are considered similar to CVE-2020-10136 in that they highlight the inherent weakness that these protocols do not validate or verify the source of a network packet. These distinct CVEs are meant to specify the different protocols in question that are vulnerable.

For reference: (CVE-2020-10136) Multiple products that implement the IP Encapsulation within IP (IPIP) standard (RFC 2003, STD 1) decapsulate and route IP-in-IP traffic without any validation, which could allow an unauthenticated remote attacker to route arbitrary traffic via an exposed network interface and lead to spoofing, access control bypass, and other unexpected network behaviors.