diff --git a/README.md b/README.md
index 7d31a90..b8f4489 100644
--- a/README.md
+++ b/README.md
@@ -101,6 +101,11 @@ Then open `http://<host>:5000` and register — **the first account becomes the
 - The app waits for the database and runs migrations (`alembic upgrade head`) automatically on start.
 - **Image tags:** `:latest` (stable, built from `main`) · `:dev` (latest `dev` build) ·
   `:<git-sha>` (immutable, for pinning / rollback).
+- **Putting it on the public internet:** there are four things to do first — close
+  registration, terminate TLS and forward `X-Forwarded-Proto`, stop publishing the app
+  port, and back up the attachment volume as well as the database. See
+  [docs/public-hosting.md](docs/public-hosting.md), which also lists what the app
+  hardens on its own and what it deliberately doesn't.
 - **Install as an app (PWA):** ThoughtSync is installable ("Add to Home Screen" / the
   browser's install button) for an app-like window. Browsers only offer install over a
   **secure context**, so put the app behind a reverse proxy terminating **HTTPS** (or reach
diff --git a/docs/public-hosting.md b/docs/public-hosting.md
new file mode 100644
index 0000000..dcf6ea8
--- /dev/null
+++ b/docs/public-hosting.md
@@ -0,0 +1,106 @@
+# Putting ThoughtSync on the public internet
+
+ThoughtSync is built to run on a LAN and works fine there with no ceremony. Exposing
+it changes the threat model: anyone can now reach the login form, and any account is
+one guessed password away from someone's whole note history.
+
+This is what the app does about that on its own, and the four things it cannot do for
+you.
+
+## Do these four things first
+
+**1. Close registration.** `allow_registration` defaults to **on**, because the first
+run of a fresh instance has to be able to create the admin account. It stays on
+afterwards. Once your own account exists, turn it off in **Settings → Access → Allow
+new registrations**, or the first stranger to find the hostname can open an account on
+your server.
+
+The first account created is always the admin, regardless of this setting — so a
+brand-new instance is never locked out of itself.
+
+**2. Terminate TLS in front of it, and forward the scheme.** The app marks the
+session cookie `Secure` and sends HSTS only when it can tell the request arrived over
+HTTPS. It looks at `X-Forwarded-Proto`, so the proxy has to set it:
+
+```
+# Traefik does this automatically. For nginx:
+proxy_set_header X-Forwarded-Proto $scheme;
+proxy_set_header X-Forwarded-For   $proxy_add_x_forwarded_for;
+```
+
+Without that header the app assumes plain HTTP and leaves the cookie unmarked — the
+conservative choice, since forcing `Secure` on an HTTP install stops the browser from
+ever sending the cookie back and silently breaks login.
+
+Once a browser has seen HSTS from your hostname it will refuse plain HTTP there for a
+year, even if the header stops. That is the point of it, but it is worth knowing
+before you put a hostname behind TLS temporarily.
+
+**3. Stop publishing the app port.** The default compose binds `0.0.0.0:5000` so LAN
+clients can reach it directly. Behind a proxy that is a second, unprotected front
+door. In `.env`:
+
+```
+THOUGHTSYNC_BIND=127.0.0.1
+```
+
+**4. Have a backup that includes the files.** Attachments are files on the
+`thoughtsync-data` volume, not rows — a `pg_dump` restores notes whose images are all
+gone. Back up both:
+
+```
+docker compose exec -T db pg_dump -U thoughtsync thoughtsync > notes.sql
+docker run --rm -v thoughtsync-data:/d -v "$PWD":/out alpine tar czf /out/media.tgz -C /d .
+```
+
+## What the app already does
+
+- **The credential endpoints are throttled.** `/api/auth/login`, `/api/auth/register`
+  and `/api/auth/device-login` count attempts against both the account and the calling
+  address, and answer `429` with a `Retry-After` once either is over budget — ten
+  failed sign-ins per account per fifteen minutes, five registrations per address per
+  hour. The account-keyed limit is the one that holds when the address is forged.
+  Checked *before* the password is verified, so a throttled attempt costs no bcrypt:
+  hashing is deliberately slow, and an unauthenticated caller who can trigger it
+  without limit has a CPU-exhaustion primitive as well as a guessing one.
+- **A failed sign-in takes the same time whether or not the account exists.** No
+  timing oracle for which emails are registered here.
+- **Every response carries a CSP** with `script-src 'self'`, `object-src 'none'` and
+  `frame-ancestors 'none'`, plus `nosniff`, a referrer policy and a permissions
+  policy. The app has no inline or third-party scripts, so this costs nothing.
+- **Link unfurling is SSRF-hardened.** Every hop is resolved and every resolved
+  address must be publicly routable before a socket is opened, and the connection is
+  made to the vetted IP so a rebind between check and connect cannot slip through. A
+  note containing `http://192.168.1.1/` cannot make your server probe your network.
+- **Attachments never render inline unless they are a known raster image.** Anything
+  else — an SVG, an HTML file — is served `Content-Disposition: attachment`, so a file
+  on a note shared with you can't run script in your session.
+- **Session cookies are `HttpOnly` and `SameSite=Lax`**, which is also what stands in
+  for CSRF protection: a `Lax` cookie is not sent on a cross-site POST.
+
+## What it does not do
+
+Know these before you decide who gets an account.
+
+- **No email verification and no password reset.** `email_verified` exists on the user
+  row and nothing sets it. A forgotten password needs a hand on the database.
+- **No second factor.** A password is the whole of it.
+- **No per-user storage quota.** Any account can upload attachments until the volume
+  is full. `max_attachment_mb` caps a single file, not a total.
+- **No audit log.** Device tokens record `last_used_at`; sign-ins are not recorded.
+
+None of these are hard blockers for an instance whose accounts are you and people you
+know. They are the reason not to hand out open registration to strangers.
+
+## The Android client
+
+The app allows plain HTTP so a self-hosted server on a LAN is usable at all — Android
+blocks cleartext by default from API 28, and `http://192.168.1.10:8000` is exactly the
+case ThoughtSync is built for. Over the public internet, link the phone to the
+**HTTPS** hostname. The sync screen shows a warning before any credential field
+whenever the address it probed was `http://`; on a public network that warning means
+what it says.
+
+The APK the server hands out is signed with the project release key, and the in-app
+updater installs over the existing app only because the signature matches. A build
+from anywhere else will not install over it.
diff --git a/src/thoughtsync/app.py b/src/thoughtsync/app.py
index 0919a90..00bd13f 100644
--- a/src/thoughtsync/app.py
+++ b/src/thoughtsync/app.py
@@ -31,6 +31,19 @@ STATIC_DIR = os.path.join(os.path.dirname(__file__), "static")
 mimetypes.add_type("application/manifest+json", ".webmanifest")
 
 
+def _is_https() -> bool:
+    """Whether this request reached us over TLS — directly, or through a proxy that
+    terminated it and said so in X-Forwarded-Proto.
+
+    Shared by the session cookie's Secure flag and by HSTS, because they are the same
+    question and answering it twice is how the two drift apart.
+    """
+    if not has_request_context():
+        return False
+    forwarded = request.headers.get("X-Forwarded-Proto", "").split(",")[0].strip().lower()
+    return forwarded == "https" or request.is_secure
+
+
 class _AutoSecureSessionInterface(SecureCookieSessionInterface):
     """Mark the session cookie `Secure` whenever the request arrived over HTTPS —
     directly, or via a TLS-terminating reverse proxy that sets X-Forwarded-Proto.
@@ -42,10 +55,7 @@ class _AutoSecureSessionInterface(SecureCookieSessionInterface):
     """
 
     def get_cookie_secure(self, app: Quart) -> bool:
-        if not has_request_context():
-            return False
-        forwarded = request.headers.get("X-Forwarded-Proto", "").split(",")[0].strip().lower()
-        return forwarded == "https" or request.is_secure
+        return _is_https()
 
 
 def create_app() -> Quart:
@@ -101,6 +111,69 @@ def create_app() -> Quart:
             with suppress(asyncio.CancelledError):
                 await task
 
+    @app.after_request
+    async def _security_headers(response):
+        """Headers a publicly-reachable instance should be sending.
+
+        None of these change how the app behaves for a legitimate caller; they narrow
+        what a browser will do if something else goes wrong.
+
+        The CSP is the substantive one. `script-src 'self'` means that even if some
+        future path did manage to reflect user text into the page, the browser would
+        refuse to run it — the app has no inline scripts and no third-party scripts,
+        so nothing legitimate is given up. The exceptions are honest ones:
+          - `style-src 'unsafe-inline'` — Vue writes inline styles itself (`v-show`
+            toggling display, TransitionGroup's FLIP setting transforms). Inline
+            STYLE is not an execution primitive the way inline script is.
+          - `img-src https: http:` — link previews render the remote og:image of
+            whatever was linked, which is an arbitrary host by definition. Both
+            schemes, because a LAN install is served over http and would otherwise
+            lose every preview image; on an https instance the browser blocks the
+            http ones as mixed content anyway, so naming it concedes nothing.
+          - `blob:`/`data:` — attachment previews and the desktop's blob URI scheme.
+
+        `frame-ancestors 'none'` replaces the older X-Frame-Options and is what stops
+        the app being framed for clickjacking; `form-action 'self'` stops a form from
+        being pointed at another origin.
+        """
+        response.headers.setdefault(
+            "Content-Security-Policy",
+            "default-src 'self'; "
+            "base-uri 'self'; "
+            "object-src 'none'; "
+            "frame-ancestors 'none'; "
+            "form-action 'self'; "
+            "script-src 'self'; "
+            "style-src 'self' 'unsafe-inline'; "
+            "img-src 'self' data: blob: https: http:; "
+            "font-src 'self' data:; "
+            "media-src 'self' blob:; "
+            "connect-src 'self'",
+        )
+        # Content-type sniffing turns a file we said was text into whatever the bytes
+        # look like. The attachment route already sets this; every other response
+        # deserves it too.
+        response.headers.setdefault("X-Content-Type-Options", "nosniff")
+        # Note titles and label names end up in the URL of a search or a label lens,
+        # and a full Referer would hand them to any site a link preview points at.
+        response.headers.setdefault("Referrer-Policy", "strict-origin-when-cross-origin")
+        # Nothing here uses a camera, a microphone or a location, so nothing embedded
+        # in a page should be able to ask on its behalf.
+        response.headers.setdefault(
+            "Permissions-Policy", "camera=(), microphone=(), geolocation=(), interest-cohort=()"
+        )
+        # HSTS only where the request already arrived over TLS — the same detection
+        # the session cookie uses. Sending it on a plain-HTTP LAN install would tell
+        # the browser to refuse the only scheme that install serves.
+        #
+        # Scoped to this host: no `includeSubDomains` and no `preload`, both of which
+        # commit domains this app does not own. A browser still remembers the policy
+        # for up to a year after the header stops being sent, which is the point of
+        # it — worth knowing before putting a hostname behind TLS temporarily.
+        if _is_https():
+            response.headers.setdefault("Strict-Transport-Security", "max-age=31536000")
+        return response
+
     @app.get("/api/health")
     async def health():
         return jsonify({"status": "ok", "version": app.config["APP_VERSION"]})
diff --git a/src/thoughtsync/auth.py b/src/thoughtsync/auth.py
index 5e9d8f6..7960d40 100644
--- a/src/thoughtsync/auth.py
+++ b/src/thoughtsync/auth.py
@@ -11,7 +11,13 @@ from .common import iso
 from .db import session_scope
 from .models.device_token import DeviceToken
 from .models.user import User
-from .security import generate_token, hash_password, hash_token, verify_password
+from .ratelimit import (
+    client_address,
+    register_by_address,
+    sign_in_by_account,
+    sign_in_by_address,
+)
+from .security import dummy_verify, generate_token, hash_password, hash_token, verify_password
 from .settings import get_setting
 
 bp = Blueprint("auth", __name__, url_prefix="/api/auth")
@@ -106,6 +112,52 @@ def require_admin(fn):
     return wrapper
 
 
+def _throttled(retry_after: int):
+    """The 429 every throttled credential route returns.
+
+    Deliberately says nothing about WHICH limit was hit or how many attempts are
+    left — that would tell someone probing whether the email they guessed exists.
+    `Retry-After` is standard and is the one thing a legitimate client (or person)
+    genuinely needs.
+    """
+    return (
+        jsonify({"error": "too many attempts — try again shortly"}),
+        429,
+        {"Retry-After": str(retry_after)},
+    )
+
+
+def _sign_in_block(email: str) -> int | None:
+    """Seconds to wait before this sign-in may be attempted, or None to proceed.
+
+    Checked BEFORE the password is verified, so a throttled attempt costs no bcrypt
+    — which is the other half of what this protects: hashing is deliberately slow,
+    and an unauthenticated caller who can trigger it without limit has a CPU
+    exhaustion primitive, not just a guessing one.
+    """
+    address = client_address()
+    waits = [
+        sign_in_by_address.retry_after(address),
+        sign_in_by_account.retry_after(email) if email else None,
+    ]
+    live = [w for w in waits if w is not None]
+    return max(live) if live else None
+
+
+def _sign_in_failed(email: str) -> None:
+    address = client_address()
+    sign_in_by_address.record(address)
+    if email:
+        sign_in_by_account.record(email)
+
+
+def _sign_in_succeeded(email: str) -> None:
+    """Clear the account's history on success. The address keeps its count: one
+    correct password does not vouch for the other attempts from there."""
+    if email:
+        sign_in_by_account.forget(email)
+
+
 @bp.post("/register")
 async def register():
     data = await request.get_json(silent=True) or {}
@@ -120,6 +172,14 @@ async def register():
     if not display_name:
         display_name = email.split("@", 1)[0]
 
+    # Counted by attempt rather than by failure: a rejected registration still cost a
+    # round trip and a uniqueness check, and on an instance with signups open the
+    # thing worth bounding is how fast accounts can appear at all.
+    wait = register_by_address.retry_after(client_address())
+    if wait is not None:
+        return _throttled(wait)
+    register_by_address.record(client_address())
+
     async with session_scope() as db:
         user_count = await db.scalar(select(func.count()).select_from(User)) or 0
         is_first = user_count == 0
@@ -150,10 +210,23 @@ async def login():
     email = (data.get("email") or "").strip().lower()
     password = data.get("password") or ""
 
+    wait = _sign_in_block(email)
+    if wait is not None:
+        return _throttled(wait)
+
     async with session_scope() as db:
         user = await db.scalar(select(User).where(User.email == email))
-        if user is None or not user.password_hash or not verify_password(password, user.password_hash):
+        if user is None or not user.password_hash:
+            # Hash anyway. Without this, "no such account" returns in microseconds
+            # while a wrong password takes bcrypt's deliberate ~100ms, and the
+            # difference is a reliable oracle for which emails have accounts here.
+            dummy_verify(password)
+            _sign_in_failed(email)
             return jsonify({"error": "invalid email or password"}), 401
+        if not verify_password(password, user.password_hash):
+            _sign_in_failed(email)
+            return jsonify({"error": "invalid email or password"}), 401
+        _sign_in_succeeded(email)
         session[SESSION_KEY] = str(user.id)
         session.permanent = True
         return jsonify(_serialize_user(user))
@@ -210,10 +283,24 @@ async def device_login():
     password = data.get("password") or ""
     if not email or not password:
         return jsonify({"error": "email and password are required"}), 400
+
+    # Same budget as the web sign-in, and the SAME counters — this route hands out a
+    # long-lived bearer token, so leaving it unthrottled would just move the guessing
+    # here from /login.
+    wait = _sign_in_block(email)
+    if wait is not None:
+        return _throttled(wait)
+
     async with session_scope() as db:
         user = await db.scalar(select(User).where(User.email == email))
-        if user is None or not user.password_hash or not verify_password(password, user.password_hash):
+        if user is None or not user.password_hash:
+            dummy_verify(password)
+            _sign_in_failed(email)
             return jsonify({"error": "invalid email or password"}), 401
+        if not verify_password(password, user.password_hash):
+            _sign_in_failed(email)
+            return jsonify({"error": "invalid email or password"}), 401
+        _sign_in_succeeded(email)
         row, token = await _issue_device_token(db, user.id, data.get("name") or "")
         await db.commit()
         return jsonify({"token": token, "device": _serialize_device(row), "user": _serialize_user(user)}), 201
diff --git a/src/thoughtsync/ratelimit.py b/src/thoughtsync/ratelimit.py
new file mode 100644
index 0000000..30f59d0
--- /dev/null
+++ b/src/thoughtsync/ratelimit.py
@@ -0,0 +1,140 @@
+"""Throttling for the endpoints that a public deployment leaves exposed.
+
+Only the credential endpoints are rate-limited: login, register, and the native
+device-link exchange. Everything else already needs a session or a device token to
+reach, so an attacker has to get through one of these three first.
+
+## Why in-process is enough here, and where that stops being true
+
+State lives in module-level dicts, so it is per-process. That is correct for how
+this image actually serves — one hypercorn worker (see the Dockerfile, and the
+same assumption the trash sweeper documents in app.py). If that ever gains
+``--workers N``, each worker would keep its own counters and the effective limit
+would multiply by N; the fix then is a shared store (the Postgres connection is
+already there), not a bigger number here.
+
+## Two keys, on purpose
+
+Every attempt is counted against BOTH the account being tried and the address it
+came from, and either one can refuse it:
+
+- **The account** is the key that matters, and the key that cannot be forged. It
+  is what stops credential stuffing against one known email, no matter how many
+  addresses the attempts arrive from.
+- **The address** bounds the damage from one source spraying many accounts. It is
+  best-effort by nature — behind a reverse proxy the client address is read from
+  ``X-Forwarded-For``, which a caller can set to anything if the app is exposed
+  directly. That is precisely why it is not the only key.
+
+Counting is by failure for the sign-in routes and by attempt for registration: a
+correct password should never move someone closer to being locked out, but every
+registration is a row in the users table whether it succeeds or not.
+"""
+from __future__ import annotations
+
+import time
+from collections import deque
+
+from quart import request
+
+# Failed sign-ins tolerated per account before it stops answering, and for how long.
+# Ten is comfortably above a person mistyping a password and far below anything that
+# makes a dictionary worth running.
+ACCOUNT_LIMIT = 10
+ACCOUNT_WINDOW_S = 15 * 60
+
+# Wider, because one address is legitimately many people: a household, an office
+# behind NAT, a phone on carrier-grade NAT.
+ADDRESS_LIMIT = 50
+ADDRESS_WINDOW_S = 15 * 60
+
+# Registration is scarcer than a sign-in — it creates a row, and on a private
+# instance the honest number of accounts anyone needs to make is one.
+REGISTER_LIMIT = 5
+REGISTER_WINDOW_S = 60 * 60
+
+# Never let the bookkeeping become the denial of service: an attacker rotating a
+# forged X-Forwarded-For could otherwise mint an unbounded number of buckets. Well
+# above any real deployment's distinct-caller count, so a legitimate instance never
+# reaches it; when it is reached the oldest buckets are dropped, which at worst
+# forgives some attempts.
+MAX_BUCKETS = 10_000
+
+
+class SlidingWindow:
+    """Counts events per key over a trailing window.
+
+    Sliding rather than a fixed window because a fixed one lets twice the limit
+    through across a boundary — 10 at 14:59 and 10 at 15:00 — which for a login
+    limiter is the difference between the number meaning something and not.
+    """
+
+    def __init__(self, limit: int, window_s: float) -> None:
+        self.limit = limit
+        self.window_s = window_s
+        self._hits: dict[str, deque[float]] = {}
+
+    def _prune(self, key: str, now: float) -> deque[float]:
+        hits = self._hits.get(key)
+        if hits is None:
+            hits = deque()
+            # Insertion-ordered, so the first key is the least recently created.
+            if len(self._hits) >= MAX_BUCKETS:
+                self._hits.pop(next(iter(self._hits)), None)
+            self._hits[key] = hits
+        cutoff = now - self.window_s
+        while hits and hits[0] <= cutoff:
+            hits.popleft()
+        return hits
+
+    def retry_after(self, key: str, now: float | None = None) -> int | None:
+        """Seconds until `key` may try again, or None while it is still under the
+        limit. Read-only — it does not count as an attempt."""
+        now = time.monotonic() if now is None else now
+        hits = self._prune(key, now)
+        if len(hits) < self.limit:
+            return None
+        # The window frees up when its OLDEST hit falls out of it.
+        return max(1, int(hits[0] + self.window_s - now) + 1)
+
+    def record(self, key: str, now: float | None = None) -> None:
+        now = time.monotonic() if now is None else now
+        self._prune(key, now).append(now)
+
+    def forget(self, key: str) -> None:
+        """Drop a key's history. Used after a successful sign-in, so someone who
+        fumbled a password twice and then got it right starts clean rather than
+        carrying those two for the next quarter of an hour."""
+        self._hits.pop(key, None)
+
+    def clear(self) -> None:
+        self._hits.clear()
+
+
+sign_in_by_account = SlidingWindow(ACCOUNT_LIMIT, ACCOUNT_WINDOW_S)
+sign_in_by_address = SlidingWindow(ADDRESS_LIMIT, ADDRESS_WINDOW_S)
+register_by_address = SlidingWindow(REGISTER_LIMIT, REGISTER_WINDOW_S)
+
+
+def client_address() -> str:
+    """The caller's address, as well as it can be known.
+
+    ``X-Forwarded-For`` is a list appended to by each hop, so the leftmost entry is
+    the original client — and also the only entry a client can choose for itself.
+    It is trusted here anyway, because the alternative behind a reverse proxy is to
+    see the proxy's address for every request on earth and rate-limit the entire
+    internet as one caller. The account-keyed limit is the one that holds when this
+    one is lied to.
+    """
+    forwarded = request.headers.get("X-Forwarded-For", "")
+    if forwarded:
+        first = forwarded.split(",")[0].strip()
+        if first:
+            return first[:64]  # bounded: this becomes a dict key
+    return (request.remote_addr or "unknown")[:64]
+
+
+def reset_all() -> None:
+    """Drop every counter. For tests — nothing in the app calls this."""
+    for window in (sign_in_by_account, sign_in_by_address, register_by_address):
+        window.clear()
diff --git a/src/thoughtsync/security.py b/src/thoughtsync/security.py
index c57ca70..fcec317 100644
--- a/src/thoughtsync/security.py
+++ b/src/thoughtsync/security.py
@@ -14,6 +14,23 @@ def hash_password(password: str) -> str:
     return bcrypt.hashpw(password.encode("utf-8")[:_MAX_BCRYPT_BYTES], bcrypt.gensalt()).decode("utf-8")
 
 
+# A real bcrypt hash of a value nobody can present, used only to spend the time a
+# verification would have. Computed once at import — generating it per call would
+# cost a gensalt+hash on top of the checkpw and make the "no such user" path SLOWER
+# than the real one, which is the same oracle pointing the other way.
+_DUMMY_HASH = bcrypt.hashpw(secrets.token_bytes(32), bcrypt.gensalt())
+
+
+def dummy_verify(password: str) -> None:
+    """Burn one password verification against a throwaway hash.
+
+    For the sign-in path when the email has no account: it makes "no such user" cost
+    what "wrong password" costs, so response time stops answering the question of
+    which emails are registered here.
+    """
+    bcrypt.checkpw(password.encode("utf-8")[:_MAX_BCRYPT_BYTES], _DUMMY_HASH)
+
+
 def verify_password(password: str, password_hash: str) -> bool:
     try:
         return bcrypt.checkpw(password.encode("utf-8")[:_MAX_BCRYPT_BYTES], password_hash.encode("utf-8"))
diff --git a/tests/test_ratelimit.py b/tests/test_ratelimit.py
new file mode 100644
index 0000000..874a749
--- /dev/null
+++ b/tests/test_ratelimit.py
@@ -0,0 +1,139 @@
+"""The credential throttle. DB-free, like the rest of this suite.
+
+The window itself is exercised directly with an injected clock, so nothing here
+sleeps: a 15-minute window tested in real time is a test nobody runs twice.
+
+The routes are exercised only as far as they get WITHOUT a database — a throttled
+request returns 429 before any session is opened, which is the whole point of
+checking the limit before the password. The happy path can't be reached here and is
+not pretended at.
+"""
+import pytest
+
+from thoughtsync import ratelimit
+from thoughtsync.app import create_app
+from thoughtsync.ratelimit import SlidingWindow
+
+
+@pytest.fixture(autouse=True)
+def _clean_counters():
+    ratelimit.reset_all()
+    yield
+    ratelimit.reset_all()
+
+
+@pytest.fixture
+def app():
+    return create_app()
+
+
+def test_under_the_limit_is_not_blocked():
+    w = SlidingWindow(limit=3, window_s=60)
+    for i in range(3):
+        assert w.retry_after("k", now=i) is None
+        w.record("k", now=i)
+    assert w.retry_after("k", now=3) is not None
+
+
+def test_window_slides_rather_than_resetting():
+    w = SlidingWindow(limit=2, window_s=60)
+    w.record("k", now=0)
+    w.record("k", now=30)
+    assert w.retry_after("k", now=31) is not None
+    # The 0s hit falls out at t=60, which frees exactly one slot — the 30s hit is
+    # still inside the window, so this is a slide and not a reset.
+    assert w.retry_after("k", now=61) is None
+    w.record("k", now=61)
+    assert w.retry_after("k", now=62) is not None
+
+
+def test_retry_after_points_past_the_oldest_hit():
+    w = SlidingWindow(limit=1, window_s=100)
+    w.record("k", now=10)
+    wait = w.retry_after("k", now=40)
+    # The hit at t=10 leaves the window at t=110, i.e. 70s away. Rounded up, never
+    # under-reported — a client that waits exactly this long must not be refused
+    # again.
+    assert wait is not None
+    assert 70 <= wait <= 72
+    assert w.retry_after("k", now=40 + wait) is None
+
+
+def test_keys_are_counted_separately():
+    w = SlidingWindow(limit=1, window_s=60)
+    w.record("a", now=0)
+    assert w.retry_after("a", now=1) is not None
+    assert w.retry_after("b", now=1) is None
+
+
+def test_forget_clears_one_key():
+    w = SlidingWindow(limit=1, window_s=60)
+    w.record("a", now=0)
+    w.record("b", now=0)
+    w.forget("a")
+    assert w.retry_after("a", now=1) is None
+    assert w.retry_after("b", now=1) is not None
+
+
+def test_bucket_count_is_bounded(monkeypatch):
+    # An attacker rotating a forged X-Forwarded-For must not be able to grow this
+    # dict without limit — the limiter cannot become the exhaustion it prevents.
+    monkeypatch.setattr(ratelimit, "MAX_BUCKETS", 8)
+    w = SlidingWindow(limit=5, window_s=60)
+    for i in range(50):
+        w.record(f"addr-{i}", now=i)
+    assert len(w._hits) <= 8
+
+
+async def test_login_starts_refusing(app):
+    client = app.test_client()
+    body = {"email": "someone@example.com", "password": "wrong-password"}
+    # Pre-load the account's counter to its limit rather than posting that many
+    # times: every real attempt would need a database to reach the password check.
+    for i in range(ratelimit.ACCOUNT_LIMIT):
+        ratelimit.sign_in_by_account.record("someone@example.com", now=float(i))
+    resp = await client.post("/api/auth/login", json=body)
+    assert resp.status_code == 429
+    assert resp.headers.get("Retry-After")
+    # The refusal says nothing about whether that account exists.
+    assert "someone@example.com" not in (await resp.get_data(as_text=True))
+
+
+async def test_device_login_shares_the_account_counter(app):
+    client = app.test_client()
+    for i in range(ratelimit.ACCOUNT_LIMIT):
+        ratelimit.sign_in_by_account.record("someone@example.com", now=float(i))
+    resp = await client.post(
+        "/api/auth/device-login",
+        json={"email": "someone@example.com", "password": "wrong-password"},
+    )
+    # Same budget as /login — otherwise guessing just moves to the route that hands
+    # out a long-lived bearer token.
+    assert resp.status_code == 429
+
+
+async def test_register_is_throttled_by_address(app):
+    client = app.test_client()
+    for i in range(ratelimit.REGISTER_LIMIT):
+        ratelimit.register_by_address.record("203.0.113.9", now=float(i))
+    resp = await client.post(
+        "/api/auth/register",
+        json={"email": "new@example.com", "password": "a-long-enough-password"},
+        headers={"X-Forwarded-For": "203.0.113.9"},
+    )
+    assert resp.status_code == 429
+
+
+async def test_client_address_prefers_the_forwarded_client(app):
+    # Behind a reverse proxy, remote_addr is the PROXY for every request on earth —
+    # keying on it would rate-limit the entire internet as one caller. The leftmost
+    # X-Forwarded-For entry is the original client.
+    async with app.test_request_context("/", headers={"X-Forwarded-For": "198.51.100.4, 10.0.0.1"}):
+        assert ratelimit.client_address() == "198.51.100.4"
+
+
+async def test_client_address_falls_back_to_the_peer(app):
+    async with app.test_request_context("/"):
+        # No proxy header: whatever the peer address is, it must be a usable key
+        # rather than an empty string sharing one bucket with everyone.
+        assert ratelimit.client_address()
diff --git a/tests/test_security_headers.py b/tests/test_security_headers.py
new file mode 100644
index 0000000..9009673
--- /dev/null
+++ b/tests/test_security_headers.py
@@ -0,0 +1,57 @@
+"""Headers every response carries once this is reachable from the internet.
+
+Asserted on /api/health because it is the one route that needs no database and no
+session — the headers are set in an after_request hook, so any response proves the
+hook, and this suite has no Postgres.
+"""
+import pytest
+
+from thoughtsync.app import create_app
+
+
+@pytest.fixture
+def app():
+    return create_app()
+
+
+async def test_content_security_policy_locks_scripts_to_self(app):
+    resp = await app.test_client().get("/api/health")
+    csp = resp.headers["Content-Security-Policy"]
+    assert "script-src 'self'" in csp
+    # The two that matter most if anything ever reflects user text into the page.
+    assert "object-src 'none'" in csp
+    assert "frame-ancestors 'none'" in csp
+    # No blanket unsafe-inline for SCRIPT — style is the only place it's conceded.
+    assert "script-src 'self' 'unsafe-inline'" not in csp
+
+
+async def test_link_preview_images_are_still_allowed(app):
+    resp = await app.test_client().get("/api/health")
+    csp = resp.headers["Content-Security-Policy"]
+    # A preview renders the og:image of an arbitrary host; both schemes, because a
+    # LAN install is served over http.
+    assert "img-src" in csp
+    assert "https:" in csp
+    assert "http:" in csp
+
+
+async def test_sniffing_and_referrer_are_pinned(app):
+    resp = await app.test_client().get("/api/health")
+    assert resp.headers["X-Content-Type-Options"] == "nosniff"
+    assert resp.headers["Referrer-Policy"] == "strict-origin-when-cross-origin"
+    assert "camera=()" in resp.headers["Permissions-Policy"]
+
+
+async def test_no_hsts_on_plain_http(app):
+    # A plain-HTTP LAN install must not be told to refuse the only scheme it serves.
+    resp = await app.test_client().get("/api/health")
+    assert "Strict-Transport-Security" not in resp.headers
+
+
+async def test_hsts_when_a_proxy_terminated_tls(app):
+    resp = await app.test_client().get("/api/health", headers={"X-Forwarded-Proto": "https"})
+    hsts = resp.headers["Strict-Transport-Security"]
+    assert "max-age=" in hsts
+    # Scoped to this host: neither of these commits domains the app doesn't own.
+    assert "includeSubDomains" not in hsts
+    assert "preload" not in hsts