gusset moves your Firefox extension settings between your own machines over a passphrase-derived encrypted channel. A flaw here can expose those settings or let an unpaired device connect, so security reports are taken seriously.

gusset is pre-1.0 and under active development. Only the latest commit on main is supported; please confirm an issue reproduces there before reporting.

Please report privately rather than opening a public issue:

• GitHub private vulnerability reporting: the "Report a vulnerability" button under this repository's Security tab, or
• email justin@justinstimatze.com.

Include what you did, what you expected, and what happened — a proof of concept helps. You'll get an acknowledgement, and a fix or explanation once the report is triaged. Please give a reasonable window to address the issue before any public disclosure.

The authoritative design and threat model live in docs/transport-and-security.md. In short:

• The only shared secret is an 8-word passphrase; all keys derive from it. Connecting requires proving knowledge of it (passphrase-derived mutual TLS) — there is no account, server, or CA to compromise.
• Carriers that move data on gusset's behalf (the device-to-device transport, and the Tier-1 rendezvous folder / Firefox Sync) only ever see ciphertext.
• Out of scope for v1: forward secrecy and per-device revocation — a lost device is handled by rotating the passphrase everywhere.

gusset is built to need no account and no server, and to keep your settings — and who-syncs-with-whom — out of any third party's hands. What that does and does not hide:

Kept private by design

• Every beacon that crosses a carrier (a --rendezvous-dir folder, or Firefox Sync's storage.sync) is sealed: the carrier sees only ciphertext, never your endpoints, device names, or settings. The settings themselves move device-to-device, never through the carrier.
• No telemetry, analytics, or update pings — the daemon makes no outbound connection you did not ask for, and the extension talks only to the local daemon over loopback (enforced by its content-security policy; it requests only the storage permission and declares no data collection).
• The passphrase is never written to the config, logged, sent, or placed in a certificate or beacon. The mutual-TLS certificate carries a fixed name (gusset-peer) and nothing identifying, and TLS 1.3 keeps even that off the wire.

Metadata that is visible, and to whom

• On a local network, gusset announces itself over mDNS so your devices can find each other. Anyone on that network can see that a gusset device is present and its random device id — which is opaque, not derived from your hostname, so it does not reveal your machine's name.
• Through Firefox Sync, the per-device beacon keys let your own Sync account see that gusset is installed and roughly how many devices you have. The values stay encrypted; Mozilla cannot read them.
• A shared rendezvous folder holds one sealed file per device, so its file count reveals your device count to anyone who can read the folder. The contents stay sealed.
• STUN is opt-in. If you pass --stun, the STUN server you name learns your public IP. To sync across networks without contacting any third party, use a shared folder (--rendezvous-dir) instead.