diff --git a/server/src/app.ts b/server/src/app.ts
index 6a7194d..89682e0 100644
--- a/server/src/app.ts
+++ b/server/src/app.ts
@@ -8,9 +8,18 @@ import { settingsRoutes } from "./routes/settings";
 
 export type Deps = { db: Db; key: Buffer; dataDir: string; llmFetch?: typeof fetch };
 const PUBLIC = new Set(["/api/health", "/api/me", "/api/setup", "/api/login"]);
+const CSP = "default-src 'self'; script-src 'self'; style-src 'self'; img-src 'self' data:; object-src 'none'; frame-ancestors 'none'";
 
 export function createApp(deps: Deps) {
   const app = new Hono();
+  app.use("*", async (c, next) => {
+    await next();
+    c.header("x-content-type-options", "nosniff");
+    if (!c.req.path.startsWith("/api/")) {
+      c.header("content-security-policy", CSP);
+      c.header("referrer-policy", "no-referrer");
+    }
+  });
   app.get("/api/health", (c) => c.json({ ok: true }));
   app.use("/api/*", async (c, next) => {
     if (PUBLIC.has(c.req.path)) return next();
diff --git a/server/test/security-headers.test.ts b/server/test/security-headers.test.ts
new file mode 100644
index 0000000..d9c9d13
--- /dev/null
+++ b/server/test/security-headers.test.ts
@@ -0,0 +1,26 @@
+import { describe, expect, test } from "bun:test";
+import { mkdtempSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { createApp } from "../src/app";
+import { openDb } from "../src/db";
+import { loadOrCreateKey } from "../src/lib/crypto";
+
+function makeApp() {
+  const dir = mkdtempSync(join(tmpdir(), "helios-"));
+  return createApp({ db: openDb(dir), key: loadOrCreateKey(dir), dataDir: dir });
+}
+
+describe("security headers", () => {
+  test("API responses: nosniff", async () => {
+    const res = await makeApp().request("/api/health");
+    expect(res.headers.get("x-content-type-options")).toBe("nosniff");
+  });
+  test("non-API responses: CSP + nosniff + referrer policy", async () => {
+    const res = await makeApp().request("/anything");
+    expect(res.headers.get("content-security-policy")).toContain("default-src 'self'");
+    expect(res.headers.get("content-security-policy")).toContain("frame-ancestors 'none'");
+    expect(res.headers.get("x-content-type-options")).toBe("nosniff");
+    expect(res.headers.get("referrer-policy")).toBe("no-referrer");
+  });
+});