thoughtsync/docs/public-hosting.md

# 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 five things first

**1. Check registration is closed.** On a fresh instance this now takes care of
itself: the first account created becomes the admin *and* closes registration behind
it, so there is no window between "my account exists" and "I remembered to turn it
off". A brand-new instance is never locked out of itself, and never left open either.

**Instances that predate this still need one manual flip.** The close fires when the
first account is created, so a server whose admin already existed keeps whatever
`allow_registration` was set to — which was **on** by default. Check **Settings →
Access → Allow new registrations** before exposing an instance you have been running
on a LAN.

To let someone else in, turn it back on, have them register, turn it off. There is no
invite system yet, so that is the mechanism.

**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. Tell it how many proxies are in front of it.** **Settings → Security → Trusted
proxy hops**, which defaults to `1` — one reverse proxy terminating TLS. Behind a CDN
as well (Cloudflare in front of your proxy) set it to `2`. It applies immediately; no
restart.

This decides which entry of `X-Forwarded-For` is believed, and it is a security
setting rather than a preference. The header grows left to right as a request
traverses, so the rightmost entries are the ones your own infrastructure wrote and
anything a caller forged sits to the left of them. Counting in from the right by the
number of proxies you actually run means a forged prefix can never be selected. Set it
too HIGH and it starts trusting entries no proxy of yours wrote; too low and several
callers share one rate-limit bucket, which is merely inconvenient.

**4. 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
```

**5. 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. The
  numbers live in **Settings → Security** — ten failed sign-ins per account per
  fifteen minutes and five sign-ups per address per hour by default — and a change
  applies to the next attempt rather than the next deploy. 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 TABLE.** Credential events — sign-ins, failures, throttle trips, new
  accounts, device tokens issued — are written to the application log and readable
  with `docker compose logs app`, which is enough to see whether anyone is knocking.
  They are not queryable, not retained beyond the container's log rotation, and not
  attributable after the fact.
- **No invites.** Adding a second person means re-opening registration while they
  sign up, then closing it again. There is no per-person token, no expiry, and no
  record of who invited whom.

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.