{
  "guidislink": false,
  "id": "https://tailscale.com/security-bulletins/#ts-2026-011",
  "link": "https://tailscale.com/security-bulletins/#ts-2026-011",
  "links": [
    {
      "href": "https://tailscale.com/security-bulletins/#ts-2026-011",
      "rel": "alternate",
      "type": "text/html"
    }
  ],
  "published": "Tue, 18 Aug 2026 00:00:00 GMT",
  "summary": "\u003cp\u003e\u003cstrong\u003e\u003cem\u003eDescription\u003c/em\u003e\u003c/strong\u003e: Insufficient validation of 4via6 subnet router destinations permitted access to host-scoped addresses, including cloud metadata and loopback services.\u003c/p\u003e\n\u003ch4\u003eWhat happened?\u003c/h4\u003e\n\u003cp\u003e\u003ca href=\"https://tailscale.com/docs/features/subnet-routers/4via6-subnets\"\u003e4via6 subnet routers\u003c/a\u003e 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.\u003c/p\u003e\n\u003cp\u003ePreviously, 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\u0027s 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.\u003c/p\u003e\n\u003cp\u003eTailscale now refuses host-scoped IPv4 destinations (loopback, link-local, multicast and broadcast) at every point that acts on an unmapped 4via6 address.\u003c/p\u003e\n\u003cp\u003eThis vulnerability is fixed in Tailscale version 1.102.3 or newer.\u003c/p\u003e\n\u003ch4\u003eWhat was the impact?\u003c/h4\u003e\n\u003cp\u003eA peer with ACL access to an advertised 4via6 route could reach host-scoped addresses from the router\u0027s network namespace. On cloud-hosted routers, this included the cloud metadata service, allowing the peer to obtain the router\u0027s instance credentials. On any router, services bound to loopback became reachable, bypassing the host firewall.\u003c/p\u003e\n\u003ch4\u003eWho was affected?\u003c/h4\u003e\n\u003cp\u003eTailnets using 4via6 subnet routing where a subnet router advertises a site prefix and a peer has ACL access to it.\u003c/p\u003e\n\u003ch4\u003eWhat do I need to do?\u003c/h4\u003e\n\u003cp\u003eIf you use 4via6 subnet routing, upgrade your subnet routers to Tailscale version 1.102.3 or newer.\u003c/p\u003e\n\u003ch4\u003eCredits\u003c/h4\u003e\n\u003cp\u003eWe would like to thank the Anthropic infrastructure security team for reporting this issue.\u003c/p\u003e",
  "summary_detail": {
    "base": "https://tailscale.com/security-bulletins/index.xml",
    "language": null,
    "type": "text/html",
    "value": "\u003cp\u003e\u003cstrong\u003e\u003cem\u003eDescription\u003c/em\u003e\u003c/strong\u003e: Insufficient validation of 4via6 subnet router destinations permitted access to host-scoped addresses, including cloud metadata and loopback services.\u003c/p\u003e\n\u003ch4\u003eWhat happened?\u003c/h4\u003e\n\u003cp\u003e\u003ca href=\"https://tailscale.com/docs/features/subnet-routers/4via6-subnets\"\u003e4via6 subnet routers\u003c/a\u003e 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.\u003c/p\u003e\n\u003cp\u003ePreviously, 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\u0027s 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.\u003c/p\u003e\n\u003cp\u003eTailscale now refuses host-scoped IPv4 destinations (loopback, link-local, multicast and broadcast) at every point that acts on an unmapped 4via6 address.\u003c/p\u003e\n\u003cp\u003eThis vulnerability is fixed in Tailscale version 1.102.3 or newer.\u003c/p\u003e\n\u003ch4\u003eWhat was the impact?\u003c/h4\u003e\n\u003cp\u003eA peer with ACL access to an advertised 4via6 route could reach host-scoped addresses from the router\u0027s network namespace. On cloud-hosted routers, this included the cloud metadata service, allowing the peer to obtain the router\u0027s instance credentials. On any router, services bound to loopback became reachable, bypassing the host firewall.\u003c/p\u003e\n\u003ch4\u003eWho was affected?\u003c/h4\u003e\n\u003cp\u003eTailnets using 4via6 subnet routing where a subnet router advertises a site prefix and a peer has ACL access to it.\u003c/p\u003e\n\u003ch4\u003eWhat do I need to do?\u003c/h4\u003e\n\u003cp\u003eIf you use 4via6 subnet routing, upgrade your subnet routers to Tailscale version 1.102.3 or newer.\u003c/p\u003e\n\u003ch4\u003eCredits\u003c/h4\u003e\n\u003cp\u003eWe would like to thank the Anthropic infrastructure security team for reporting this issue.\u003c/p\u003e"
  },
  "title": "TS-2026-011",
  "title_detail": {
    "base": "https://tailscale.com/security-bulletins/index.xml",
    "language": null,
    "type": "text/plain",
    "value": "TS-2026-011"
  }
}