diff --git a/nix/modules/hive-forge.nix b/nix/modules/hive-forge.nix
index 73639373..64ad4ee4 100644
--- a/nix/modules/hive-forge.nix
+++ b/nix/modules/hive-forge.nix
@@ -87,12 +87,22 @@ in
 
     openFirewall = lib.mkOption {
       type = lib.types.bool;
-      default = true;
+      default = false;
+      example = true;
       description = ''
-        Open `httpPort` + `sshPort` in the host firewall. Off when
-        the forge should only be reachable from inside the host.
-        (The container shares host netns, so this is the only
-        firewall layer that matters.)
+        Open `httpPort` + `sshPort` in the host firewall. Off by
+        default (#651, secure-by-default): the forge is reachable
+        from the host + every agent container via `localhost` either
+        way (shared netns), so the firewall opens only matter for
+        access from outside the host. Flip to `true` when you want
+        the operator's browser / external git clients to hit the
+        forge directly. (The container shares host netns, so this
+        is the only firewall layer that matters.)
+
+        **Breaking change as of #651**: this used to default to
+        `true`. If you relied on the old default for external
+        reach, add `services.hyperhive.forge.openFirewall = true;`
+        to your host config before rebuilding.
       '';
     };
   };
diff --git a/nix/modules/hive-gateway.nix b/nix/modules/hive-gateway.nix
index c4c288c2..f051fe0d 100644
--- a/nix/modules/hive-gateway.nix
+++ b/nix/modules/hive-gateway.nix
@@ -82,11 +82,23 @@ in
 
     openFirewall = lib.mkOption {
       type = lib.types.bool;
-      default = true;
+      default = false;
+      example = true;
       description = ''
-        Open `port` in the host firewall. Off when the gateway should
-        only be reachable from inside the host (e.g. behind another
-        reverse proxy that handles TLS termination).
+        Open `port` in the host firewall. Off by default (#651,
+        secure-by-default). Flip to `true` to expose the gateway to
+        the operator's browser / external clients — required for any
+        out-of-host reach, since the agents themselves talk to
+        hive-c0re via the per-agent unix sockets and don't need the
+        nginx vhost. Leave off when running behind another reverse
+        proxy (e.g. caddy / traefik on the host) that handles TLS
+        termination + forwards to `port`.
+
+        **Breaking change as of #651**: this used to default to
+        `true`. If you relied on the old default for external reach
+        (the common case — the gateway is the operator's primary
+        entry point), add `services.hyperhive.gateway.openFirewall = true;`
+        to your host config before rebuilding.
       '';
     };
 
diff --git a/nix/modules/hive-matrix.nix b/nix/modules/hive-matrix.nix
index 750c1ea8..5be59b66 100644
--- a/nix/modules/hive-matrix.nix
+++ b/nix/modules/hive-matrix.nix
@@ -98,12 +98,21 @@ in
 
     openFirewall = lib.mkOption {
       type = lib.types.bool;
-      default = true;
+      default = false;
+      example = true;
       description = ''
-        Open `httpPort` in the host firewall. Off when the
-        homeserver should only be reachable from inside the host
-        (e.g. while bringing the integration up before announcing
-        it to other hives).
+        Open `httpPort` in the host firewall. Off by default (#651,
+        secure-by-default): the homeserver is reachable from the
+        host + every agent container via `localhost` either way
+        (shared netns), so the firewall open only matters for
+        access from outside the host. Flip to `true` when announcing
+        the homeserver to other hives or when an external matrix
+        client needs to reach the client-server API directly.
+
+        **Breaking change as of #651**: this used to default to
+        `true`. If you relied on the old default for external reach,
+        add `services.hyperhive.matrix.openFirewall = true;` to
+        your host config before rebuilding.
 
         Note: federation (the matrix-spec well-known port 8448) is
         intentionally not opened here. tuwunel serves the federation