diff --git a/web/src/lib/__tests__/idleLogout.test.ts b/web/src/lib/__tests__/idleLogout.test.ts
new file mode 100644
index 0000000..3f594b7
--- /dev/null
+++ b/web/src/lib/__tests__/idleLogout.test.ts
@@ -0,0 +1,54 @@
+import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
+import { startIdleLogout, stopIdleLogout, IDLE_TIMEOUT_MS } from "@/lib/idleLogout";
+
+describe("idle sign-out on an untrusted device", () => {
+  beforeEach(() => vi.useFakeTimers());
+  afterEach(() => {
+    stopIdleLogout();
+    vi.useRealTimers();
+  });
+
+  it("signs out after five minutes of nothing happening", () => {
+    const expire = vi.fn();
+    startIdleLogout(expire);
+    expect(IDLE_TIMEOUT_MS).toBe(5 * 60 * 1000);
+
+    vi.advanceTimersByTime(IDLE_TIMEOUT_MS - 1);
+    expect(expire).not.toHaveBeenCalled();
+    vi.advanceTimersByTime(1);
+    expect(expire).toHaveBeenCalledTimes(1);
+  });
+
+  it("starts the clock again on any sign of a person", () => {
+    const expire = vi.fn();
+    startIdleLogout(expire);
+
+    vi.advanceTimersByTime(IDLE_TIMEOUT_MS - 1000);
+    window.dispatchEvent(new Event("keydown"));
+    vi.advanceTimersByTime(IDLE_TIMEOUT_MS - 1000);
+    expect(expire).not.toHaveBeenCalled();
+
+    vi.advanceTimersByTime(1000);
+    expect(expire).toHaveBeenCalledTimes(1);
+  });
+
+  it("fires once, not repeatedly, and stops listening afterwards", () => {
+    const expire = vi.fn();
+    startIdleLogout(expire);
+    vi.advanceTimersByTime(IDLE_TIMEOUT_MS * 3);
+    expect(expire).toHaveBeenCalledTimes(1);
+
+    // A late event must not resurrect a timer for a session that has ended.
+    window.dispatchEvent(new Event("keydown"));
+    vi.advanceTimersByTime(IDLE_TIMEOUT_MS * 2);
+    expect(expire).toHaveBeenCalledTimes(1);
+  });
+
+  it("stops cleanly, so a trusted sign-in is never signed out", () => {
+    const expire = vi.fn();
+    startIdleLogout(expire);
+    stopIdleLogout();
+    vi.advanceTimersByTime(IDLE_TIMEOUT_MS * 2);
+    expect(expire).not.toHaveBeenCalled();
+  });
+});
diff --git a/web/src/lib/__tests__/storage.test.ts b/web/src/lib/__tests__/storage.test.ts
new file mode 100644
index 0000000..b598e79
--- /dev/null
+++ b/web/src/lib/__tests__/storage.test.ts
@@ -0,0 +1,105 @@
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import {
+  accountKey,
+  clearAllData,
+  clearSignedInData,
+  isDeviceTrusted,
+  loadJson,
+  loadRaw,
+  saveJson,
+  setDeviceTrusted,
+} from "@/lib/storage";
+
+/**
+ * The gate is a privacy boundary rather than a convenience, so it is tested
+ * from both sides: that a trusted device still works exactly as it did, and
+ * that an untrusted one leaves nothing to find.
+ */
+describe("device-trusted storage", () => {
+  let store: Map<string, string>;
+
+  beforeEach(() => {
+    store = new Map();
+    Object.defineProperty(globalThis, "localStorage", {
+      configurable: true,
+      value: {
+        get length() {
+          return store.size;
+        },
+        key: (i: number) => [...store.keys()][i] ?? null,
+        getItem: (k: string) => store.get(k) ?? null,
+        setItem: (k: string, v: string) => void store.set(k, v),
+        removeItem: (k: string) => void store.delete(k),
+      },
+    });
+    setDeviceTrusted(false);
+  });
+
+  afterEach(() => {
+    setDeviceTrusted(false);
+    Reflect.deleteProperty(globalThis, "localStorage");
+  });
+
+  it("writes nothing at all when the device is not trusted", () => {
+    saveJson("settings", { theme: "dark" });
+    saveJson(accountKey("acct1", "recent"), [{ email: "someone@example.com" }]);
+    expect([...store.keys()].filter((k) => k !== "ihasmail:deviceTrusted")).toEqual([]);
+  });
+
+  it("does not read residue left by an earlier trusted session", () => {
+    setDeviceTrusted(true);
+    saveJson(accountKey("acct1", "recent"), [{ email: "someone@example.com" }]);
+    setDeviceTrusted(false);
+    // The bytes are still on disk until a purge; the gate must not serve them.
+    expect(loadRaw(accountKey("acct1", "recent"), [])).toEqual([]);
+  });
+
+  it("round-trips normally on a trusted device", () => {
+    setDeviceTrusted(true);
+    saveJson("settings", { theme: "dark" });
+    expect(loadJson("settings", { theme: "light", accent: "blue" })).toEqual({ theme: "dark", accent: "blue" });
+    expect(isDeviceTrusted()).toBe(true);
+  });
+
+  it("remembers trust across a reload, so a trusted device still paints from cache", () => {
+    setDeviceTrusted(true);
+    expect(store.get("ihasmail:deviceTrusted")).toBe("1");
+    setDeviceTrusted(false);
+    expect(store.has("ihasmail:deviceTrusted")).toBe(false);
+  });
+
+  it("clears the account's data on sign-out but keeps the deliberate exceptions", () => {
+    setDeviceTrusted(true);
+    saveJson("settings", { theme: "dark" });
+    saveJson("mbx-expanded", { a: true });
+    saveJson(accountKey("acct1", "recent"), [{ email: "someone@example.com" }]);
+    store.set("ihasmail:lastUser", "me@example.com");
+    store.set("ihasmail:pushDeviceId", "ihasmail-abc");
+
+    clearSignedInData();
+
+    expect(store.has("ihasmail:settings")).toBe(false);
+    expect(store.has("ihasmail:mbx-expanded")).toBe(false);
+    expect(store.has("ihasmail:acct1:recent")).toBe(false);
+    // Kept on purpose: prefills sign-in, and only a trusted device wrote it.
+    expect(store.get("ihasmail:lastUser")).toBe("me@example.com");
+    expect(store.get("ihasmail:pushDeviceId")).toBe("ihasmail-abc");
+  });
+
+  it("clears everything, lastUser included, for an untrusted sign-in", () => {
+    setDeviceTrusted(true);
+    saveJson("settings", { theme: "dark" });
+    store.set("ihasmail:lastUser", "me@example.com");
+
+    clearAllData();
+
+    expect([...store.keys()]).toEqual([]);
+  });
+
+  it("leaves keys belonging to anything else alone", () => {
+    setDeviceTrusted(true);
+    store.set("someone-elses-key", "keep me");
+    clearAllData();
+    expect(store.get("someone-elses-key")).toBe("keep me");
+  });
+});
diff --git a/web/src/lib/__tests__/theme.test.ts b/web/src/lib/__tests__/theme.test.ts
index f73ce3e..930d42d 100644
--- a/web/src/lib/__tests__/theme.test.ts
+++ b/web/src/lib/__tests__/theme.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from "vitest";
 import { DEFAULT_SETTINGS, DEVICE_KEYS, acceptRemote, isDarkTheme, syncedPart, toggleTarget, useSettings, type Theme } from "@/store/settings";
-import { loadJson, saveJson } from "@/lib/storage";
+import { loadJson, saveJson, setDeviceTrusted } from "@/lib/storage";
 
 /**
  * "ihasmail" is a dark theme wearing ihasmail.org's palette. Everything that
@@ -60,9 +60,14 @@ describe("the default theme", () => {
         removeItem: (k: string) => void store.delete(k),
       },
     });
+    // Reads and writes are gated on device trust now, and the gate defaults to
+    // closed. These tests are about `loadJson`'s merge, so open it and put it
+    // back -- an untrusted device is covered by storage.test.ts instead.
+    setDeviceTrusted(true);
     try {
       fn();
     } finally {
+      setDeviceTrusted(false);
       Reflect.deleteProperty(globalThis, "localStorage");
     }
   };
diff --git a/web/src/lib/idleLogout.ts b/web/src/lib/idleLogout.ts
new file mode 100644
index 0000000..cabb49c
--- /dev/null
+++ b/web/src/lib/idleLogout.ts
@@ -0,0 +1,59 @@
+/**
+ * Sign out an untrusted device after a few minutes of inactivity.
+ *
+ * This exists because the alternative does not work. Asking someone to
+ * remember to sign out relies on the person, which is the part you cannot rely
+ * on when the machine is not theirs — and a browser cannot help: custom
+ * `beforeunload` text was removed years ago, and no event fires at all for the
+ * case that actually matters, which is walking away from a signed-in screen.
+ *
+ * A timer needs nobody's cooperation, so that is what this is.
+ *
+ * Trusted devices are left alone entirely: the whole point of saying a machine
+ * is yours is not being signed out of it.
+ */
+const IDLE_MS = 5 * 60 * 1000;
+
+/** Coarse enough not to fire constantly, broad enough to catch a person reading. */
+const ACTIVITY = ["mousedown", "keydown", "touchstart", "scroll", "focus"] as const;
+
+let timer: ReturnType<typeof setTimeout> | null = null;
+let onExpire: (() => void) | null = null;
+
+function arm(): void {
+  if (timer) clearTimeout(timer);
+  timer = setTimeout(() => {
+    timer = null;
+    const fn = onExpire;
+    stopIdleLogout();
+    fn?.();
+  }, IDLE_MS);
+}
+
+/**
+ * Reading a long message is not idleness, but it produces no events either.
+ * Visibility is the honest signal available: a hidden tab is one nobody is
+ * looking at, so the clock keeps running; showing it again is activity.
+ */
+function onVisibility(): void {
+  if (document.visibilityState === "visible") arm();
+}
+
+export function startIdleLogout(expire: () => void): void {
+  stopIdleLogout();
+  onExpire = expire;
+  for (const ev of ACTIVITY) window.addEventListener(ev, arm, { passive: true, capture: true });
+  document.addEventListener("visibilitychange", onVisibility);
+  arm();
+}
+
+export function stopIdleLogout(): void {
+  if (timer) clearTimeout(timer);
+  timer = null;
+  onExpire = null;
+  for (const ev of ACTIVITY) window.removeEventListener(ev, arm, { capture: true });
+  document.removeEventListener("visibilitychange", onVisibility);
+}
+
+/** Exported for tests, which should not wait five real minutes. */
+export const IDLE_TIMEOUT_MS = IDLE_MS;
diff --git a/web/src/lib/storage.ts b/web/src/lib/storage.ts
index 8c5e64e..852a400 100644
--- a/web/src/lib/storage.ts
+++ b/web/src/lib/storage.ts
@@ -1,6 +1,93 @@
+/**
+ * Local storage, gated on whether this device is trusted.
+ *
+ * Everything here is a *cache* or a screen preference — the real copy lives in
+ * the account's JMAP Files (see `settingsSync`). That makes it safe to write
+ * nothing at all, which is what an untrusted device does: on a shared or public
+ * machine the cost of a stale first frame is nothing beside leaving someone's
+ * address book on it.
+ *
+ * Reads are gated as well as writes. A machine that was trusted once still has
+ * the residue, and honouring it would let a previous session's data surface in
+ * a later untrusted one.
+ */
 const PREFIX = "ihasmail:";
 
+/**
+ * Kept when a session ends. Everything else is cleared, so a key added later
+ * is forgotten by default rather than by nobody having thought about it.
+ *
+ * - `lastUser` is a deliberate convenience: it prefills the sign-in field, and
+ *   it is only ever written by a trusted device in the first place.
+ * - `deviceTrusted` is how the next boot knows to read at all.
+ * - `pushDeviceId` is a random id for this browser, so re-subscribing replaces
+ *   rather than accumulates. The subscription itself is removed on sign-out.
+ */
+const KEEP_ON_SIGN_OUT = ["lastUser", "deviceTrusted", "pushDeviceId"];
+
+const TRUST_KEY = `${PREFIX}deviceTrusted`;
+
+/**
+ * Read at module load rather than waiting for the session, so a trusted device
+ * still paints its first frame from cache. An untrusted one has nothing to
+ * read, so there is nothing to wait for.
+ */
+let trusted = (() => {
+  try {
+    return localStorage.getItem(TRUST_KEY) === "1";
+  } catch {
+    return false;
+  }
+})();
+
+export function isDeviceTrusted(): boolean {
+  return trusted;
+}
+
+/** Set from the session's `remember` flag, which is the answer given at sign-in. */
+export function setDeviceTrusted(value: boolean): void {
+  trusted = value;
+  try {
+    if (value) localStorage.setItem(TRUST_KEY, "1");
+    else localStorage.removeItem(TRUST_KEY);
+  } catch {
+    /* private mode: the in-memory flag still holds for this tab */
+  }
+}
+
+/** Every `ihasmail:` key currently present, without the prefix. */
+function ownKeys(): string[] {
+  const out: string[] = [];
+  try {
+    for (let i = 0; i < localStorage.length; i++) {
+      const k = localStorage.key(i);
+      if (k && k.startsWith(PREFIX)) out.push(k.slice(PREFIX.length));
+    }
+  } catch {
+    /* ignore */
+  }
+  return out;
+}
+
+/**
+ * Drop what this browser was holding for a signed-in account. Called on every
+ * sign-out, trusted or not: handing a laptop to someone else is the same
+ * exposure as a public machine, only quieter.
+ */
+export function clearSignedInData(): void {
+  for (const key of ownKeys()) {
+    if (KEEP_ON_SIGN_OUT.includes(key)) continue;
+    removeKey(key);
+  }
+}
+
+/** Everything, `lastUser` included — for signing in to a device we do not trust. */
+export function clearAllData(): void {
+  for (const key of ownKeys()) removeKey(key);
+}
+
 export function loadJson<T>(key: string, fallback: T): T {
+  if (!trusted) return fallback;
   try {
     const raw = localStorage.getItem(PREFIX + key);
     if (raw == null) return fallback;
@@ -11,6 +98,7 @@ export function loadJson<T>(key: string, fallback: T): T {
 }
 
 export function loadRaw<T>(key: string, fallback: T): T {
+  if (!trusted) return fallback;
   try {
     const raw = localStorage.getItem(PREFIX + key);
     if (raw == null) return fallback;
@@ -21,6 +109,7 @@ export function loadRaw<T>(key: string, fallback: T): T {
 }
 
 export function saveJson(key: string, value: unknown): void {
+  if (!trusted) return;
   try {
     localStorage.setItem(PREFIX + key, JSON.stringify(value));
   } catch {
diff --git a/web/src/lib/webpush.ts b/web/src/lib/webpush.ts
index 0432d03..2c8cd3f 100644
--- a/web/src/lib/webpush.ts
+++ b/web/src/lib/webpush.ts
@@ -19,6 +19,7 @@
  */
 import { CAP, client } from "@/jmap/client";
 import type { GetResponse, Id, SetResponse } from "@/jmap/types";
+import { isDeviceTrusted } from "@/lib/storage";
 
 export const VAPID_CAP = "urn:ietf:params:jmap:webpush-vapid";
 export const EMAILPUSH_CAP = "urn:ietf:params:jmap:emailpush";
@@ -93,6 +94,10 @@ export function encodeKey(buffer: ArrayBuffer | null): string {
 /** A stable id for this browser, so a re-subscribe replaces rather than piles up. */
 export function deviceClientId(): string {
   const KEY = "ihasmail:pushDeviceId";
+  // An untrusted device gets a per-session id instead of a stored one. It is
+  // the same trade private mode already makes below: re-subscribing will not
+  // reuse it, which costs nothing when push is refused there anyway.
+  if (!isDeviceTrusted()) return `ihasmail-${crypto.randomUUID()}`;
   try {
     const existing = localStorage.getItem(KEY);
     if (existing) return existing;
diff --git a/web/src/lib/webpushEnable.ts b/web/src/lib/webpushEnable.ts
index 8017977..c84f222 100644
--- a/web/src/lib/webpushEnable.ts
+++ b/web/src/lib/webpushEnable.ts
@@ -6,6 +6,7 @@
  * permission prompt, none of which exists under a test runner.
  */
 import { CAP } from "@/jmap/client";
+import { isDeviceTrusted } from "@/lib/storage";
 import { useSession } from "@/store/session";
 import { useMail } from "@/store/mail";
 import {
@@ -67,6 +68,12 @@ export async function enableWebPush(): Promise<{ ok: true } | { ok: false; reaso
   if (Notification.permission === "denied") {
     return { ok: false, reason: "Notifications are blocked for this site in your browser's settings." };
   }
+  // A subscription outlives the tab and belongs to the account, not the
+  // session -- so on a machine the user has told us is not theirs, it would go
+  // on delivering their mail to it long after they had gone.
+  if (!isDeviceTrusted()) {
+    return { ok: false, reason: "Background notifications need a device you have marked as your own. Sign in again with \u201CThis is my own device\u201D ticked." };
+  }
   const key = applicationServerKey();
   if (!key) return { ok: false, reason: "This mail server does not publish a push key." };
 
diff --git a/web/src/store/contacts.ts b/web/src/store/contacts.ts
index 2e07567..f461821 100644
--- a/web/src/store/contacts.ts
+++ b/web/src/store/contacts.ts
@@ -1,4 +1,5 @@
 import { create } from "zustand";
+import { accountKey, loadRaw, saveJson } from "@/lib/storage";
 import { CAP, client, setErrorMessage } from "@/jmap/client";
 import type { AddressBook, ContactCard, EmailAddress, GetResponse, Id, Principal, QueryResponse, SetResponse } from "@/jmap/types";
 import { contactDisplayName, contactEmails, sortKey } from "@/lib/contacts";
@@ -441,7 +442,7 @@ export const useContacts = create<ContactsState>((set, get) => ({
     const next = [...addrs.filter((a) => a.email), ...cur.filter((r) => !addrs.some((a) => a.email.toLowerCase() === r.email.toLowerCase()))].slice(0, 200);
     set({ recent: next });
     try {
-      localStorage.setItem(`ihasmail:${get().accountId}:recent`, JSON.stringify(next));
+      saveJson(accountKey(get().accountId, "recent"), next);
     } catch {
       /* ignore */
     }
@@ -466,7 +467,7 @@ useSession.subscribe((s) => {
     const accountId = s.accountFor(CAP.contacts);
     let recent: EmailAddress[] = [];
     try {
-      recent = JSON.parse(localStorage.getItem(`ihasmail:${accountId}:recent`) ?? "[]") as EmailAddress[];
+      recent = loadRaw<EmailAddress[]>(accountKey(accountId, "recent"), []);
     } catch {
       /* ignore */
     }
diff --git a/web/src/store/session.ts b/web/src/store/session.ts
index db82aa3..df13cea 100644
--- a/web/src/store/session.ts
+++ b/web/src/store/session.ts
@@ -7,6 +7,8 @@ import { setServerLocale } from "@/lib/datetime";
 import { flushSettingsPush, stopSettingsSync } from "@/lib/settingsSync";
 import { reloadIfServerRebuilt } from "@/lib/staleBuild";
 import { unsubscribeThisDevice } from "@/lib/webpush";
+import { clearAllData, clearSignedInData, setDeviceTrusted } from "@/lib/storage";
+import { startIdleLogout, stopIdleLogout } from "@/lib/idleLogout";
 
 export type AuthStatus = "loading" | "anonymous" | "authenticated";
 
@@ -81,6 +83,11 @@ export const useSession = create<SessionState>((set, get) => ({
     } catch {
       /* ignore */
     }
+    stopIdleLogout();
+    // Unconditional. The push subscription above is removed for exactly this
+    // reason -- that a browser left holding someone's mail is somebody else's
+    // problem next -- and the address book cached here is the same argument.
+    clearSignedInData();
     client.session = null;
     set({ status: "anonymous", session: null, accountId: null });
   },
@@ -112,6 +119,19 @@ export const useSession = create<SessionState>((set, get) => ({
 function applySession(s: JmapSession, set: (p: Partial<SessionState>) => void) {
   client.session = s;
   setServerLocale(s.ihasmail?.userLocale);
+  // `remember` is the answer to "is this device yours", given at sign-in and
+  // carried on the session -- so a reload arrives at the same answer without
+  // the client storing it, which on an untrusted device it could not do anyway.
+  const trusted = Boolean(s.ihasmail?.remember);
+  setDeviceTrusted(trusted);
+  if (trusted) {
+    stopIdleLogout();
+  } else {
+    // Residue from an earlier trusted session on this machine is exactly what
+    // an untrusted sign-in is asking us not to keep.
+    clearAllData();
+    startIdleLogout(() => void useSession.getState().logout());
+  }
   const accountId = s.primaryAccounts[CAP.mail] ?? Object.keys(s.accounts)[0] ?? null;
   set({ status: "authenticated", session: s, accountId, error: null });
 }
@@ -119,6 +139,8 @@ function applySession(s: JmapSession, set: (p: Partial<SessionState>) => void) {
 client.onUnauthenticated(() => {
   push.stop();
   stopSettingsSync();
+  stopIdleLogout();
+  clearSignedInData();
   client.session = null;
   // Ask before showing the sign-in form rather than after. A deploy is the
   // usual reason to be signed out here, and reloading a form someone has
diff --git a/web/src/views/Login.tsx b/web/src/views/Login.tsx
index 402fd98..52c3aae 100644
--- a/web/src/views/Login.tsx
+++ b/web/src/views/Login.tsx
@@ -22,7 +22,7 @@ export function LoginPage() {
   const [username, setUsername] = useState(() => localStorage.getItem("ihasmail:lastUser") ?? "");
   const [password, setPassword] = useState("");
   const [showPw, setShowPw] = useState(false);
-  const [remember, setRemember] = useState(true);
+  const [trustDevice, setTrustDevice] = useState(false);
   const [busy, setBusy] = useState(false);
   const [error, setError] = useState<string | null>(null);
 
@@ -34,8 +34,8 @@ export function LoginPage() {
     try {
       // No two-factor code: the field is not on this form until the flow works
       // end to end, and the server treats an absent code as none given.
-      await login(username.trim(), password, "", remember);
-      localStorage.setItem("ihasmail:lastUser", username.trim());
+      await login(username.trim(), password, "", trustDevice);
+      if (trustDevice) localStorage.setItem("ihasmail:lastUser", username.trim());
     } catch (err) {
       if (err instanceof ApiError) {
         if (err.code === "invalid_credentials") {
@@ -74,10 +74,15 @@ export function LoginPage() {
             </button>
           </div>
         </div>
-        <label className="check" style={{ marginBottom: 12 }}>
-          <input type="checkbox" checked={remember} onChange={(e) => setRemember(e.target.checked)} />
-          <span>Keep me signed in on this device</span>
+        <label className="check" style={{ marginBottom: 4 }}>
+          <input type="checkbox" checked={trustDevice} onChange={(e) => setTrustDevice(e.target.checked)} />
+          <span>This is my own device</span>
         </label>
+        <p className="hint" style={{ marginBottom: 12 }}>
+          {trustDevice
+            ? "Stay signed in, and keep settings and recent addresses on this computer."
+            : "Signed out after 5 minutes of inactivity, and nothing is kept on this computer. Leave this unticked on a shared or public one."}
+        </p>
         <button className="btn btn-primary btn-lg btn-block" type="submit" disabled={busy}>
           {busy ? <span className="spinner" style={{ borderTopColor: "#fff" }} /> : <LogIn size={18} />}
           {busy ? "Signing in…" : "Sign in"}