CVE-2025-23018 Overview
CVE-2025-23018 is a network protocol vulnerability affecting IPv4-in-IPv6 and IPv6-in-IPv6 tunneling as specified in RFC 2473. The vulnerability exists because these tunneling protocols do not require validation or verification of the source of a network packet, allowing an attacker to spoof and route arbitrary traffic via an exposed network interface. This vulnerability is similar to CVE-2020-10136, which affected IP-in-IP tunneling protocols.
Critical Impact
Attackers can exploit this vulnerability to bypass network security controls, spoof traffic sources, and potentially route malicious traffic through vulnerable systems acting as unwitting relay points.
Affected Products
- IETF IPv6 (RFC 2473 implementations)
- Systems implementing IPv4-in-IPv6 tunneling
- Systems implementing IPv6-in-IPv6 tunneling
Discovery Timeline
- 2025-01-14 - CVE CVE-2025-23018 published to NVD
- 2025-11-03 - Last updated in NVD database
Technical Details for CVE-2025-23018
Vulnerability Analysis
This vulnerability stems from a fundamental design weakness in the IPv4-in-IPv6 and IPv6-in-IPv6 tunneling protocols as defined in IETF RFC 2473. The protocol specification does not mandate source address validation for encapsulated packets traversing tunnel endpoints. This architectural gap allows attackers with network access to craft specially formatted encapsulated packets that appear to originate from trusted sources.
The attack requires high complexity to execute successfully, but does not require authentication or user interaction. When exploited, the vulnerability can affect the confidentiality, integrity, and availability of systems beyond the initially compromised component, as indicated by the changed scope in the vulnerability assessment. Detailed technical analysis of this class of tunneling vulnerabilities is available in the USENIX 2025 Tunnels Paper.
Root Cause
The root cause is the absence of mandatory source validation mechanisms in the RFC 2473 tunneling specification. Tunnel endpoints that accept and process encapsulated packets without verifying the legitimacy of the outer packet's source address can be abused to relay traffic. This represents an improper verification of source of communication channel (CWE-940).
Attack Vector
The attack is network-based and requires the attacker to have visibility to a vulnerable tunnel endpoint. The attacker crafts encapsulated IPv4-in-IPv6 or IPv6-in-IPv6 packets with spoofed source addresses. When these packets reach a vulnerable tunnel endpoint, they are decapsulated and forwarded based on the inner packet's destination, effectively allowing the attacker to:
- Route traffic through the vulnerable system to reach otherwise inaccessible networks
- Bypass access control lists and firewall rules that rely on source address verification
- Conduct denial-of-service amplification attacks by reflecting traffic through vulnerable endpoints
- Potentially perform man-in-the-middle attacks by positioning themselves in the traffic path
The Top10VPN Tunneling Protocol Vulnerability Research provides additional context on real-world exploitation scenarios. The CERT Vulnerability Advisory #199397 contains official guidance on this vulnerability class.
Detection Methods for CVE-2025-23018
Indicators of Compromise
- Unexpected encapsulated IPv6 traffic arriving at tunnel endpoints from untrusted sources
- Anomalous traffic patterns showing IPv4-in-IPv6 or IPv6-in-IPv6 packets with mismatched source addresses
- Increased volume of tunneled traffic from IP addresses not associated with legitimate tunnel peers
- Network traffic logs showing decapsulated packets being forwarded to unexpected destinations
Detection Strategies
- Implement deep packet inspection at network boundaries to analyze encapsulated traffic and verify source legitimacy
- Deploy IDS/IPS rules to detect IPv4-in-IPv6 and IPv6-in-IPv6 packets from unauthorized sources
- Configure network monitoring tools to alert on tunneling protocol traffic from IP ranges outside of expected tunnel peer addresses
- Enable logging on all tunnel endpoints and correlate with known authorized peer lists
Monitoring Recommendations
- Monitor tunnel endpoint interfaces for traffic volume anomalies that could indicate exploitation
- Implement flow analysis to track the ratio of legitimate versus potentially spoofed tunnel traffic
- Set up alerts for tunnel traffic originating from geographic regions where no authorized peers exist
- Review firewall and router logs regularly for rejected or suspicious tunneling protocol packets
How to Mitigate CVE-2025-23018
Immediate Actions Required
- Audit all systems and network devices for exposed IPv4-in-IPv6 and IPv6-in-IPv6 tunnel endpoints
- Implement strict access control lists to limit which source addresses can send traffic to tunnel endpoints
- Disable unnecessary tunneling interfaces on systems that do not require this functionality
- Configure firewalls to block tunneling protocol traffic from untrusted networks
Patch Information
As this is a protocol-level vulnerability in RFC 2473, there is no single vendor patch available. Mitigation depends on implementing proper source validation controls at the network and system level. Organizations should consult their specific network equipment and operating system vendors for guidance on implementing source address validation for tunnel endpoints. The IETF RFC 2473 Document provides the base protocol specification that requires augmentation with security controls.
Workarounds
- Implement ingress filtering (BCP 38/RFC 2827) to prevent spoofed packets from entering the network
- Configure tunnel endpoints to only accept traffic from explicitly authorized IP addresses using ACLs
- Deploy IPsec or other authentication mechanisms for tunnel traffic to ensure source authenticity
- Consider disabling IPv4-in-IPv6 and IPv6-in-IPv6 tunneling where not strictly required for operations
# Example: Linux iptables rules to restrict tunnel traffic sources
# Allow tunnel traffic only from authorized peer 2001:db8::1
ip6tables -A INPUT -p ipv6 -s 2001:db8::1 -j ACCEPT
ip6tables -A INPUT -p ipv6 -j DROP
# Example: Disable automatic tunneling interfaces if not needed
sysctl -w net.ipv6.conf.all.disable_ipv6=1
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
