docs(security): correct the M3 pod-to-node control #403

SECURITY.md

@@ -202,11 +202,17 @@ checks remain necessary after deployment.
   migration and a maintenance window. See WORKAROUNDS.md.
 - **Remote consoles:** the libvirt VNC consoles listen on `127.0.0.1` only, and the shared firewall
   drops 5900-5910 on every host. Reach a console through an SSH tunnel.
-- **Pod-to-host firewall:** pod CIDR sources (`10.0.0.0/8`, `fd00::/104`) are accepted on the k3s
-  nodes, but the credential-bearing host ports are denied first (SSH, NFS, Postgres, restic, VNC,
-  Immich, and the exporter/monitoring ports). The broad accept stays so Cilium and in-cluster
-  service traffic keep working; a full port allowlist is future work. Ports 80/443 are deliberately
-  not denied, so in-cluster calls through the ingress are not broken.
+- **Pod-to-host firewall:** two independent layers deny pod access to the credential-bearing host
+  ports (SSH, NFS, Postgres, restic, VNC, Immich, exporter/monitoring). The
+  `deny-pod-to-node-admin-ports` `CiliumClusterwideNetworkPolicy` in
+  [k8s-gitops#52](https://github.com/ryan4yin/k8s-gitops/pull/52) denies pod egress to
+  `host`/`remote-node` on those ports at the pod's own node; the k3s nodes' nftables rule drops the
+  same ports for pod CIDR sources (`10.0.0.0/8`, `fd00::/104`). The nftables rule only matches when
+  the pod IP survives to the destination, which requires Cilium's eBPF datapath: kube-proxy's
+  iptables masquerade rewrote pod -> node traffic to the source node IP and defeated it. That is why
+  k3s runs `--disable-kube-proxy` ([#404](https://github.com/ryan4yin/nix-config/pull/404)) and
+  Cilium runs `kubeProxyReplacement` with `bpf.masquerade`. Verified live on all nodes: pod ->
+  node:22 is dropped while kubelet, ingress (80/443 stay open), ClusterIP and DNS keep working.
 
 Only selected pod security fields, RBAC rules/binding metadata, listeners and export metadata were
 queried. Secret contents and raw kubeconfigs were not inspected. No live access policy, workload,