TS-2026-011

Description: Insufficient validation of 4via6 subnet router destinations permitted access to host-scoped addresses, including cloud metadata and loopback services. What happened? 4via6 subnet routers embed an IPv4 address within an IPv6 address. When a peer sends traffic to such an address, the router unmaps the embedded IPv4 address and forwards the traffic to it. Previously, this forwarding path did not check the class of the embedded IPv4 address. A peer with ACL access to an advertised 4via6 site could therefore reach host-scoped addresses as seen from the router, including its cloud metadata service and services bound to loopback. Tailscale's packet filter already drops link-local destinations, but for 4via6 traffic it only sees the outer IPv6 address, so that check previously did not run against the embedded IPv4 destination. Tailscale now refuses host-scoped IPv4 destinations (loopback, link-local, multicast and broadcast) at every point that acts on an unmapped 4via6 address. This vulnerability is fixed in Tailscale version 1.102.3 or newer. What was the impact? A peer with ACL access to an advertised 4via6 route could reach host-scoped addresses from the router's network namespace. On cloud-hosted routers, this included the cloud metadata service, allowing the peer to obtain the router's instance credentials. On any router, services bound to loopback became reachable, bypassing the host firewall. Who was affected? Tailnets using 4via6 subnet routing where a subnet router advertises a site prefix and a peer has ACL access to it. What do I need to do? If you use 4via6 subnet routing, upgrade your subnet routers to Tailscale version 1.102.3 or newer. Credits We would like to thank the Anthropic infrastructure security team for reporting this issue.