diff --git a/.env.example b/.env.example
index ea89325e..865e0d2d 100644
--- a/.env.example
+++ b/.env.example
@@ -49,7 +49,8 @@ ECONOMIC_API_APP_SECRET_TOKEN=
 
 # Edge broker defaults for shell relay and gateway dispatch.
 EDGE_BROKER_URL=http://edge-broker:4300
-EDGE_BROKER_SHARED_SECRET=truckwash-edge-dev
+EDGE_BROKER_SHARED_SECRET=
+EDGE_AUTH_MODE=strict
 
 # Redis credentials
 REDIS_CONFIG_HOST=redis
diff --git a/docker-compose.example.yml b/docker-compose.example.yml
index 1e415a4b..51580bdd 100644
--- a/docker-compose.example.yml
+++ b/docker-compose.example.yml
@@ -52,9 +52,10 @@ services:
       dockerfile: services/edge-broker/Dockerfile
     container_name: edge-broker
     environment:
-      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev}
+      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET}
+      EDGE_AUTH_MODE: ${EDGE_AUTH_MODE:-strict}
     ports:
-      - "4300:4300"
+      - "127.0.0.1:4300:4300"
 
   caddy:
     image: caddy:2.7.6-alpine
@@ -98,7 +99,7 @@ services:
     environment:
       AUTO_COMPOSER_INSTALL: "true"
       EDGE_BROKER_URL: ${EDGE_BROKER_URL:-http://edge-broker:4300}
-      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev}
+      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET}
     volumes:
       - ./services/nginx/app:/var/www/html
       - ./services/edge-agent/dist:/services/edge-agent/dist:ro
@@ -120,7 +121,7 @@ services:
     environment:
       AUTO_COMPOSER_INSTALL: "false"
       EDGE_BROKER_URL: ${EDGE_BROKER_URL:-http://edge-broker:4300}
-      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev}
+      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET}
     volumes:
       - ./services/nginx/app:/var/www/html
       - ./services/edge-agent/dist:/services/edge-agent/dist:ro
diff --git a/docker-compose.yml b/docker-compose.yml
index ccbcc3d2..b8b565b1 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -86,9 +86,10 @@ services:
       dockerfile: services/edge-broker/Dockerfile
     container_name: edge-broker
     environment:
-      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev}
+      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET}
+      EDGE_AUTH_MODE: ${EDGE_AUTH_MODE:-strict}
     ports:
-      - "4300:4300"
+      - "127.0.0.1:4300:4300"
 
   caddy:
     image: caddy:2.7.6-alpine
@@ -213,7 +214,7 @@ services:
     environment:
       AUTO_COMPOSER_INSTALL: "true"
       EDGE_BROKER_URL: ${EDGE_BROKER_URL:-http://edge-broker:4300}
-      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev}
+      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET}
     volumes:
       - ./services/nginx/app:/var/www/html
       - ./services/edge-agent/dist:/services/edge-agent/dist:ro
@@ -234,7 +235,7 @@ services:
     environment:
       AUTO_COMPOSER_INSTALL: "false"
       EDGE_BROKER_URL: ${EDGE_BROKER_URL:-http://edge-broker:4300}
-      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev}
+      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET}
     volumes:
       - ./services/nginx/app:/var/www/html
       - ./services/edge-agent/dist:/services/edge-agent/dist:ro
@@ -255,7 +256,7 @@ services:
     environment:
       AUTO_COMPOSER_INSTALL: "false"
       EDGE_BROKER_URL: ${EDGE_BROKER_URL:-http://edge-broker:4300}
-      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev}
+      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET}
     volumes:
       - ./services/nginx/app:/var/www/html
       - ./services/edge-agent/dist:/services/edge-agent/dist:ro
@@ -276,7 +277,7 @@ services:
     environment:
       AUTO_COMPOSER_INSTALL: "false"
       EDGE_BROKER_URL: ${EDGE_BROKER_URL:-http://edge-broker:4300}
-      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev}
+      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET}
     volumes:
       - ./services/nginx/app:/var/www/html
       - ./services/edge-agent/dist:/services/edge-agent/dist:ro
@@ -297,7 +298,7 @@ services:
     environment:
       AUTO_COMPOSER_INSTALL: "false"
       EDGE_BROKER_URL: ${EDGE_BROKER_URL:-http://edge-broker:4300}
-      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev}
+      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET}
     volumes:
       - ./services/nginx/app:/var/www/html
       - ./services/edge-agent/dist:/services/edge-agent/dist:ro
@@ -318,7 +319,7 @@ services:
     environment:
       AUTO_COMPOSER_INSTALL: "false"
       EDGE_BROKER_URL: ${EDGE_BROKER_URL:-http://edge-broker:4300}
-      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev}
+      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET}
     volumes:
       - ./services/nginx/staging:/var/www/html
       - ./services/edge-agent/dist:/services/edge-agent/dist:ro
@@ -339,7 +340,7 @@ services:
     environment:
       AUTO_COMPOSER_INSTALL: "false"
       EDGE_BROKER_URL: ${EDGE_BROKER_URL:-http://edge-broker:4300}
-      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev}
+      EDGE_BROKER_SHARED_SECRET: ${EDGE_BROKER_SHARED_SECRET}
     volumes:
       - ./services/nginx/app:/var/www/html
       - ./services/edge-agent/dist:/services/edge-agent/dist:ro
diff --git a/services/edge-broker/server.mjs b/services/edge-broker/server.mjs
index 70eed202..8a2fdf35 100644
--- a/services/edge-broker/server.mjs
+++ b/services/edge-broker/server.mjs
@@ -25,15 +25,24 @@ function jsonResponse(res, statusCode, body) {
 }
 
 export function createBrokerServer(options = {}) {
-  const authMode = options.authMode || process.env.EDGE_AUTH_MODE || "stub";
+  const authMode = options.authMode || process.env.EDGE_AUTH_MODE || "strict";
   const sharedSecret = options.sharedSecret ?? process.env.EDGE_BROKER_SHARED_SECRET ?? "";
   const commandTimeoutMs = options.commandTimeoutMs ?? 10000;
   const agents = new Map();
   const pendingCommands = new Map();
   const browserSessions = new Map();
+  const isStubMode = authMode === "stub";
 
-  const validateAgent = options.validateAgent || (async ({ gatewayId }) => ({ id: gatewayId }));
-  const validateShellSession = options.validateShellSession || (async ({ token }) => ({ id: token, gateway_id: 1, reason: "stub" }));
+  const validateAgent = options.validateAgent || (isStubMode
+    ? (async ({ gatewayId }) => ({ id: gatewayId }))
+    : (async () => {
+      throw new Error("Unauthorized");
+    }));
+  const validateShellSession = options.validateShellSession || (isStubMode
+    ? (async ({ token }) => ({ id: token, gateway_id: 1, reason: "stub" }))
+    : (async () => {
+      throw new Error("Unauthorized");
+    }));
   const closeShellSession = options.closeShellSession || (async () => ({}));
 
   const markBrowserSessionsClosed = (gatewayId, reason) => {
@@ -113,9 +122,7 @@ export function createBrokerServer(options = {}) {
           socket.destroy();
           return;
         }
-        if (authMode !== "stub") {
-          await validateAgent({ gatewayId, token, headers: req.headers });
-        }
+        await validateAgent({ gatewayId, token, headers: req.headers });
         wss.handleUpgrade(req, socket, head, (ws) => {
           ws.gatewayId = gatewayId;
           agents.set(gatewayId, ws);
@@ -130,9 +137,7 @@ export function createBrokerServer(options = {}) {
           socket.destroy();
           return;
         }
-        const session = authMode === "stub"
-          ? await validateShellSession({ token })
-          : await validateShellSession({ token, headers: req.headers });
+        const session = await validateShellSession({ token, headers: req.headers });
 
         wss.handleUpgrade(req, socket, head, (ws) => {
           ws.sessionToken = token;
diff --git a/services/edge-broker/test/broker.test.mjs b/services/edge-broker/test/broker.test.mjs
index 4c6cce92..7607f10d 100644
--- a/services/edge-broker/test/broker.test.mjs
+++ b/services/edge-broker/test/broker.test.mjs
@@ -24,6 +24,18 @@ function waitForClose(socket) {
   });
 }
 
+function waitForCloseOrError(socket) {
+  return new Promise((resolve) => {
+    const onDone = () => {
+      socket.off("error", onDone);
+      socket.off("close", onDone);
+      resolve();
+    };
+    socket.once("error", onDone);
+    socket.once("close", onDone);
+  });
+}
+
 async function waitFor(predicate, { timeoutMs = 1000, intervalMs = 10, description = "condition" } = {}) {
   const deadline = Date.now() + timeoutMs;
 
@@ -215,3 +227,14 @@ test("broker closes browser shell sessions when the agent disconnects before she
 
   await broker.close();
 });
+
+test("broker defaults to strict auth when no validators are configured", async () => {
+  const broker = createBrokerServer();
+  const address = await broker.listen(0);
+  const port = address.port;
+
+  const agent = new WebSocket(`ws://127.0.0.1:${port}/ws/agent?gatewayId=701&token=agent-token`);
+  await waitForCloseOrError(agent);
+
+  await broker.close();
+});
diff --git a/services/edge-broker/test/config.test.mjs b/services/edge-broker/test/config.test.mjs
index 1783b44b..8c10e658 100644
--- a/services/edge-broker/test/config.test.mjs
+++ b/services/edge-broker/test/config.test.mjs
@@ -24,19 +24,19 @@ test("traefik does not expose a dedicated public edge broker port", () => {
   assert.doesNotMatch(traefikSource, /edge-broker:\s*\n\s*address:\s*":4300"/);
 });
 
-test("base docker compose exposes the edge broker service on port 4300", () => {
+test("base docker compose restricts edge broker host binding to loopback", () => {
   assert.match(baseComposeSource, /\bedge-broker:\b/);
-  assert.match(baseComposeSource, /edge-broker:\s*\n[\s\S]*?\n\s+ports:\s*\n\s+- "4300:4300"/);
+  assert.match(baseComposeSource, /edge-broker:\s*\n[\s\S]*?\n\s+ports:\s*\n\s+- "127\.0\.0\.1:4300:4300"/);
 });
 
-test("example docker compose exposes the edge broker service on port 4300", () => {
+test("example docker compose restricts edge broker host binding to loopback", () => {
   assert.match(exampleComposeSource, /\bedge-broker:\b/);
-  assert.match(exampleComposeSource, /edge-broker:\s*\n[\s\S]*?\n\s+ports:\s*\n\s+- "4300:4300"/);
+  assert.match(exampleComposeSource, /edge-broker:\s*\n[\s\S]*?\n\s+ports:\s*\n\s+- "127\.0\.0\.1:4300:4300"/);
 });
 
-test("php services receive broker websocket environment defaults", () => {
+test("compose config does not provide insecure broker secret defaults", () => {
   for (const composeSource of [baseComposeSource, exampleComposeSource]) {
     assert.match(composeSource, /EDGE_BROKER_URL:\s*\$\{EDGE_BROKER_URL:-http:\/\/edge-broker:4300\}/);
-    assert.match(composeSource, /EDGE_BROKER_SHARED_SECRET:\s*\$\{EDGE_BROKER_SHARED_SECRET:-truckwash-edge-dev\}/);
+    assert.match(composeSource, /EDGE_BROKER_SHARED_SECRET:\s*\$\{EDGE_BROKER_SHARED_SECRET\}/);
   }
 });