diff --git a/.changeset/whole-needles-remain.md b/.changeset/whole-needles-remain.md
new file mode 100644
index 0000000..a845151
--- /dev/null
+++ b/.changeset/whole-needles-remain.md
@@ -0,0 +1,2 @@
+---
+---
diff --git a/server/app.ts b/server/app.ts
index 66c89b1..71e9415 100644
--- a/server/app.ts
+++ b/server/app.ts
@@ -287,6 +287,13 @@ export function createApp({
   maxHoldConnections = DEFAULT_MAX_HOLD_CONNECTIONS,
 }: AppOptions) {
   const app = new Hono();
+  // `?key=` bootstraps cookie auth, so never let a board URL disclose that
+  // credential to another origin through an outbound Referer header. Set this
+  // before auth so denied and public routes carry the same policy.
+  app.use("*", (c, next) => {
+    c.header("Referrer-Policy", "no-referrer");
+    return next();
+  });
   const bus = new EventBus();
   if (onEvent) {
     bus.subscribe((event) => {
diff --git a/test/api.test.ts b/test/api.test.ts
index a843aaa..db1ac20 100644
--- a/test/api.test.ts
+++ b/test/api.test.ts
@@ -1171,6 +1171,20 @@ test("auth token guards mutating routes when configured", async () => {
   assert.equal(viaCookie.status, 200);
 });
 
+test("Referrer-Policy protects public, denied, and authenticated board responses", async () => {
+  const app = makeApp("secret");
+  const responses = await Promise.all([
+    app.request("/guide"),
+    app.request("/"),
+    app.request("/?key=secret"),
+    app.request("/api/sessions"),
+  ]);
+
+  for (const response of responses) {
+    assert.equal(response.headers.get("referrer-policy"), "no-referrer");
+  }
+});
+
 async function readSseUntil(res: Response, needle: string, abort?: () => void): Promise<string> {
   assert.ok(res.body);
   const reader = res.body.getReader();
diff --git a/test/workerIntegration.integration.ts b/test/workerIntegration.integration.ts
index 3b16e6a..43d7c89 100644
--- a/test/workerIntegration.integration.ts
+++ b/test/workerIntegration.integration.ts
@@ -169,6 +169,7 @@ test(
     assert.equal(rendered.status, 200);
     assert.match(await rendered.text(), /worker-marker/);
     assert.equal(rendered.headers.get("content-security-policy"), "sandbox allow-scripts");
+    assert.equal(rendered.headers.get("referrer-policy"), "no-referrer");
     assert.equal(rendered.headers.get("x-content-type-options"), "nosniff");
     assert.match(rendered.headers.get("cache-control") ?? "", /immutable/);