diff --git a/README.md b/README.md
index 467653c..8aa58f2 100644
--- a/README.md
+++ b/README.md
@@ -162,6 +162,7 @@ ALLOW_SIGNUPS=false
 ```
 - **First run:** there is no default account and no password in the environment. The first visitor to the login page is asked to create an account. **Do this right after starting the container**, before anyone else can reach it.
 - **More accounts:** sign-up is off by default. Set `ALLOW_SIGNUPS=true` to show a **Create one** link on the login page, and set it back to `false` once everyone has an account. Creating the first account is always allowed.
+- **Sessions:** a login lasts 7 days. **Logout** ends that session on the server, so a copy of its cookie stops working too; other devices stay signed in. Sessions (basic and OIDC) are listed in `/config/sessions.json`, so a restart keeps you signed in.
 - **Changing a password:** click **Password** in the top bar. This signs out that user's other sessions.
 - **Storage:** accounts live in `/config/users.json`, with passwords as salted scrypt hashes and the file readable only by the container user.
 - **Forgotten password:** remove that user's entry from `/config/users.json` (or delete the file to start over) and restart the container.
diff --git a/root/app/backend/auth.py b/root/app/backend/auth.py
index 916db09..094b946 100644
--- a/root/app/backend/auth.py
+++ b/root/app/backend/auth.py
@@ -230,6 +230,70 @@ def check_basic_credentials(username: str, password: str) -> bool:
     ok = verify_password_hash(password, user["hash"] if user else _DUMMY_HASH)
     return user is not None and ok
 
+# ----------------- Sessions -----------------
+
+# Session cookies are signed, so the server can trust them without storing them, but
+# then logging out could only delete the browser's copy: any other copy stayed valid
+# until it expired. Each cookie therefore names a session id that must still be listed
+# here, and logging out removes it. Kept on disk so a restart doesn't sign everyone out.
+SESSIONS_FILE = CONFIG_DIR / "sessions.json"
+_sessions_lock = threading.Lock()
+
+def _load_sessions() -> Dict[str, float]:
+    """{session id: expiry time}. A missing or damaged file just means no sessions."""
+    try:
+        data = json.loads(SESSIONS_FILE.read_text())
+        if isinstance(data, dict):
+            return {sid: float(exp) for sid, exp in data.items() if isinstance(sid, str)}
+    except FileNotFoundError:
+        pass
+    except (OSError, ValueError, TypeError):
+        logger.warning("%s is unreadable; existing sessions must log in again", SESSIONS_FILE)
+    return {}
+
+def _save_sessions(sessions: Dict[str, float]):
+    """Caller must hold _sessions_lock."""
+    CONFIG_DIR.mkdir(parents=True, exist_ok=True)
+    tmp = SESSIONS_FILE.with_name(SESSIONS_FILE.name + ".tmp")
+    fd = os.open(tmp, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)
+    with os.fdopen(fd, "w") as f:
+        json.dump(sessions, f)
+        f.flush()
+        os.fsync(f.fileno())
+    os.replace(tmp, SESSIONS_FILE)
+
+_sessions = _load_sessions()
+
+def start_session() -> str:
+    """A new session id, valid for MAX_AGE."""
+    global _sessions
+    sid = secrets.token_urlsafe(24)
+    now = time.time()
+    with _sessions_lock:
+        sessions = {k: v for k, v in _sessions.items() if v > now}  # drop expired ones
+        sessions[sid] = now + MAX_AGE
+        _save_sessions(sessions)
+        _sessions = sessions
+    return sid
+
+def session_active(sid: Optional[str]) -> bool:
+    return bool(sid) and _sessions.get(sid, 0) > time.time()
+
+def end_session(token: Optional[str]):
+    """Revoke the session a cookie names, so no copy of that cookie works any more."""
+    global _sessions
+    if not token:
+        return
+    try:
+        sid = serializer.loads(token, max_age=MAX_AGE).get("sid")
+    except (BadSignature, SignatureExpired, AttributeError):
+        return
+    with _sessions_lock:
+        if sid in _sessions:
+            sessions = {k: v for k, v in _sessions.items() if k != sid}
+            _save_sessions(sessions)
+            _sessions = sessions
+
 # ----------------- Login rate limiting -----------------
 
 LOGIN_MAX_FAILURES = 10
@@ -329,7 +393,7 @@ def _cookie_secure(request: Optional[Request]) -> bool:
 def create_session_cookie(response: Response, user_data: dict, request: Optional[Request] = None):
     if user_data.get("auth_mode") == "basic":
         user_data = {**user_data, "pwv": user_epoch(user_data.get("username", ""))}
-    token = serializer.dumps(user_data)
+    token = serializer.dumps({**user_data, "sid": start_session()})
     response.set_cookie(
         key=COOKIE_NAME,
         value=token,
@@ -397,7 +461,8 @@ def get_current_user(request: Request) -> Dict[str, Any]:
             # is removed from users.json
             epoch = user_epoch(data.get("username", ""))
             stale = AUTH_MODE == "basic" and (epoch is None or data.get("pwv") != epoch)
-            if data.get("auth_mode") == AUTH_MODE and not stale:
+            # Also rejects cookies issued before sessions had ids: one more login after upgrading
+            if data.get("auth_mode") == AUTH_MODE and not stale and session_active(data.get("sid")):
                 return {
                     "authenticated": True,
                     "username": data.get("username", "user"),
diff --git a/root/app/backend/main.py b/root/app/backend/main.py
index 475b1c4..994a0d0 100644
--- a/root/app/backend/main.py
+++ b/root/app/backend/main.py
@@ -35,6 +35,8 @@
     client_ip,
     create_session_cookie,
     clear_session_cookie,
+    end_session,
+    COOKIE_NAME,
     create_state_cookie,
     verify_state_cookie,
     clear_state_cookie
@@ -332,7 +334,9 @@ async def oidc_callback(request: Request, code: Optional[str] = None, state: Opt
 # POST only: the session cookie is SameSite=Lax, which still rides along when another site
 # navigates here, so a GET logout let any page sign the user out
 @app.post("/auth/logout")
-async def logout():
+def logout(request: Request):
+    # Ends the session itself, not just this browser's copy of the cookie
+    end_session(request.cookies.get(COOKIE_NAME))
     # Cookie must be cleared on the response actually returned, not an injected one.
     # 303 so the browser follows up with a GET of the login page.
     response = RedirectResponse("/login.html", status_code=status.HTTP_303_SEE_OTHER)
diff --git a/tests/test_account.py b/tests/test_account.py
index 8b13152..f84079f 100644
--- a/tests/test_account.py
+++ b/tests/test_account.py
@@ -170,3 +170,61 @@ def test_logout_by_post_clears_the_session(app):
     assert res.headers["location"] == "/login.html"
     assert auth.COOKIE_NAME in res.headers.get("set-cookie", "")
     assert client.get("/auth/status").json()["authenticated"] is False
+
+
+# ----------------- Server-side sessions -----------------
+
+def test_a_copied_cookie_stops_working_after_logout(app):
+    """Logging out ends the session, not just this browser's copy of the cookie."""
+    main, auth = app
+    client, res = signup(main)
+    cookie = client.cookies.get(auth.COOKIE_NAME)
+    copy = TestClient(main.app, cookies={auth.COOKIE_NAME: cookie})
+    assert copy.get("/auth/status").json()["authenticated"] is True
+    client.post("/auth/logout", follow_redirects=False)
+    assert copy.get("/auth/status").json()["authenticated"] is False
+
+
+def test_logging_out_one_session_keeps_the_others(app):
+    main, auth = app
+    signup(main)
+    laptop, phone = login(main), login(main)
+    laptop.post("/auth/logout", follow_redirects=False)
+    assert phone.get("/auth/status").json()["authenticated"] is True
+
+
+def test_sessions_survive_a_restart(app):
+    main, auth = app
+    client, _ = signup(main)
+    cookie = client.cookies.get(auth.COOKIE_NAME)
+    for name in ("auth", "main"):
+        sys.modules.pop(name, None)
+    main2 = importlib.import_module("main")
+    sys.modules["auth"].AUTH_MODE = main2.AUTH_MODE = "basic"
+    copy = TestClient(main2.app, cookies={auth.COOKIE_NAME: cookie})
+    assert copy.get("/auth/status").json()["authenticated"] is True
+
+
+def test_a_validly_signed_cookie_without_a_session_is_rejected(app):
+    """Cookies issued before sessions had ids; also any cookie whose session ended."""
+    main, auth = app
+    signup(main)
+    token = auth.serializer.dumps({"username": "alice", "auth_mode": "basic",
+                                   "pwv": auth.user_epoch("alice")})
+    client = TestClient(main.app, cookies={auth.COOKIE_NAME: token})
+    assert client.get("/auth/status").json()["authenticated"] is False
+
+
+def test_expired_sessions_are_dropped_from_the_file(app, monkeypatch):
+    main, auth = app
+    signup(main)
+    real_time = auth.time.time
+    monkeypatch.setattr(auth.time, "time", lambda: real_time() + auth.MAX_AGE + 60)
+    login(main)
+    assert len(auth._load_sessions()) == 1  # only the new one
+
+
+def test_a_damaged_sessions_file_means_no_sessions(app):
+    main, auth = app
+    auth.SESSIONS_FILE.write_text("{not json")
+    assert auth._load_sessions() == {}