Tsinghua University & Zhongguancun Laboratory Tsinghua University Southeast University The Internet Control Message Protocol for IPv6 (ICMPv6) is essential for network diagnostics but is vulnerable to off-path spoofing attacks, especially when error messages relate to stateless transport protocols like UDP. An attacker can forge these messages to degrade performance or enable Man-in-the-Middle attacks. This document proposes a robust, stateless challenge-response mechanism to authenticate ICMPv6 error messages. Traditional stateful challenge mechanisms are vulnerable to state-exhaustion Denial-of-Service (DoS) attacks. To avoid this, the proposed solution is inspired by TCP SYN-Cookies, eliminating the need to store per-challenge state by using cryptographic computation. It limits state management to minimal flags on existing sockets or a bounded probabilistic data structure. This approach effectively authenticates ICMPv6 error messages while inherently resisting both off-path spoofing and state-exhaustion DoS attacks, thus improving the robustness of ICMPv6.