CVE-2025-23019 Overview
CVE-2025-23019 is a protocol vulnerability affecting IPv6-in-IPv4 tunneling mechanisms as specified in RFC 4213. This vulnerability allows an attacker to spoof and route traffic via an exposed network interface, potentially enabling traffic interception, redirection, and manipulation of network communications.
The vulnerability stems from insufficient verification of the source address in tunneled packets, allowing attackers to inject malicious traffic into tunnel endpoints. This affects any system implementing IPv6-in-IPv4 transition mechanisms without proper authentication controls.
Critical Impact
Attackers can exploit this vulnerability to spoof traffic, potentially bypassing security controls, intercepting sensitive data, or launching man-in-the-middle attacks against systems using IPv6-in-IPv4 tunneling.
Affected Products
- IETF IPv6 (IPv6-in-IPv4 tunneling implementations per RFC 4213)
- Systems implementing automatic or configured IPv6-in-IPv4 tunneling
- Network devices with exposed tunnel interfaces
Discovery Timeline
- 2025-01-14 - CVE-2025-23019 published to NVD
- 2025-11-03 - Last updated in NVD database
Technical Details for CVE-2025-23019
Vulnerability Analysis
The vulnerability exists in the IPv6-in-IPv4 tunneling mechanism defined in RFC 4213. This transition mechanism encapsulates IPv6 packets within IPv4 packets to enable IPv6 connectivity over IPv4-only network segments. The core issue is that the protocol specification does not mandate strict source address verification for incoming tunneled packets.
When a tunnel endpoint receives an encapsulated IPv6 packet, it processes the inner IPv6 payload based on destination addressing without adequately validating that the outer IPv4 source address corresponds to a legitimate tunnel peer. This allows an attacker with network access to craft specially formed packets that appear to originate from trusted tunnel endpoints.
The attack can be executed remotely over the network, though exploitation requires high complexity as the attacker must understand the target's tunneling configuration and be positioned to inject packets. Successful exploitation can impact confidentiality, integrity, and availability of network communications, with potential to affect systems beyond the initially targeted tunnel endpoint due to the changed scope characteristic.
Root Cause
The root cause is categorized under CWE-940 (Improper Verification of Source of a Communication Channel). The RFC 4213 specification does not require tunnel endpoints to authenticate or strictly verify the source of incoming encapsulated packets. This design decision prioritized interoperability and ease of deployment over security, creating an inherent trust relationship that attackers can exploit.
The lack of mandatory source address filtering or cryptographic authentication in the base protocol allows attackers to inject packets that will be decapsulated and processed as legitimate IPv6 traffic.
Attack Vector
The attack vector is network-based and requires the attacker to be able to send packets to the target's tunnel interface. The exploitation scenario typically involves:
- The attacker identifies a target system with an exposed IPv6-in-IPv4 tunnel endpoint
- The attacker crafts IPv4 packets containing encapsulated IPv6 payloads with spoofed source addresses
- These packets are sent to the target's tunnel interface
- The tunnel endpoint decapsulates and processes the malicious IPv6 traffic as legitimate
- The attacker can then redirect, intercept, or manipulate traffic flows
The attack does not require user interaction and can be performed without prior authentication. For detailed technical analysis, refer to the USENIX 2025 Tunnels Research Paper and CERT Vulnerability Note #199397.
Detection Methods for CVE-2025-23019
Indicators of Compromise
- Unexpected IPv4 encapsulated packets arriving on tunnel interfaces from untrusted source addresses
- Anomalous routing table changes affecting IPv6 destinations
- Network traffic patterns showing IPv6 traffic originating from unauthorized IPv4 sources
- Increased tunnel interface traffic from addresses not matching configured tunnel peers
Detection Strategies
- Implement network monitoring to detect IPv4 packets with protocol 41 (IPv6 encapsulation) from unexpected sources
- Deploy intrusion detection rules to identify spoofed tunnel traffic based on source address anomalies
- Monitor for routing changes that could indicate traffic redirection attacks
- Analyze packet captures at tunnel endpoints for source address mismatches between configured peers and actual traffic
Monitoring Recommendations
- Enable logging on all tunnel interfaces to capture source addresses of incoming encapsulated traffic
- Configure alerts for tunnel traffic originating from addresses outside the expected peer list
- Implement flow analysis to detect unusual patterns in encapsulated IPv6 traffic
- Regularly audit tunnel configurations and compare against expected network topology
How to Mitigate CVE-2025-23019
Immediate Actions Required
- Audit all network interfaces and disable IPv6-in-IPv4 tunneling where not explicitly required
- Implement strict ingress filtering (BCP38/RFC 2827) at network boundaries to prevent spoofed packets
- Configure firewall rules to restrict tunnel endpoint access to known, trusted peer addresses only
- Isolate tunnel interfaces from untrusted networks where possible
Patch Information
This is a protocol-level vulnerability in RFC 4213 specifications. There is no traditional software patch available. Mitigation requires configuration changes and deployment of additional security controls. Organizations should consult the IETF RFC 4213 Document for protocol specifications and the Top10VPN Tunneling Protocol Study for remediation guidance.
Vendors implementing IPv6-in-IPv4 tunneling should provide configuration options for strict source address validation and consider implementing IPsec or other authentication mechanisms for tunnel traffic.
Workarounds
- Disable automatic IPv6 tunneling mechanisms (6to4, ISATAP, Teredo) on systems that do not require IPv6 transition
- Deploy IPsec to authenticate and encrypt tunnel traffic, ensuring only authorized peers can communicate
- Implement strict source address filtering at tunnel endpoints using access control lists
- Consider migrating to native IPv6 connectivity where possible, eliminating the need for transition mechanisms
# Configuration example - Disable automatic tunneling interfaces on Linux
# Disable 6to4 tunneling
ip tunnel del sit0 2>/dev/null || true
echo 1 > /proc/sys/net/ipv6/conf/all/disable_ipv6_tunnels
# Block protocol 41 (IPv6 encapsulation) from untrusted sources using iptables
iptables -A INPUT -p 41 -s ! trusted_peer_ip -j DROP
# Example: Restrict tunnel interface to specific peer only
ip tunnel change sit1 local your_ip remote trusted_peer_ip mode sit
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
