10 — Security Model

The server is security-sensitive: it spawns processes, writes to PTYs, and accepts automation commands. Threat model: malicious or buggy local content (terminal output, hook payloads, plugin scripts, notification text) and other local users; no remote attacker in scope for the MVP (no network listener).

Boundaries & requirements

• Local-only: Unix socket only, under $XDG_RUNTIME_DIR/signaltty/ (dir 0700, socket 0600, user-owned). No TCP listener by default; any future remote bridge must be explicit, authenticated, and documented.
• Peer identity: SO_PEERCRED check — refuse connections whose uid ≠ server uid, even if filesystem perms were loosened.
• Input validation: every IPC param validated (id format, size clamps, argv non-empty + executable lookup, signal allowlist, base64 bounds, 16 MiB line cap). Malformed input → error, never panic.
• Kill scope: pane.signal defaults to the child pid only; process-group kill requires explicit group: true and only targets the pane's own pgid. No arbitrary-pid signalling.
• Spawn scope: pane.spawn runs as the server user with cwd constrained to directories the user can access; env overrides pass an allowlist + blocklist (LD_PRELOAD, LD_LIBRARY_PATH stripped unless explicitly permitted in server config).
• Automation boundaries: pane.input/pane.read are the orchestration primitives — available to any local client of the user's own socket. Document that any process running as the user can drive panes; this matches tmux semantics. Future macaroons/scopes (read-only automation tokens) are a Phase 4 hardening item, not MVP.
• Notification sanitization: title/body stripped of C0/C1 controls (except \n\t), OSC/CSI sequences neutralized, length-capped (title 200, body 2000 chars) before storage, IPC fan-out, or desktop delivery. Desktop actions carry only opaque ids — clicking never executes embedded commands.
• Hook payloads: treated as untrusted JSON; size-capped (256 KiB), parsed strictly, unknown fields ignored; hook shims run with user privileges and must be user-installed (installer never overwrites user hook files — merges/appends with markers).
• Terminal output: the server's OSC scanner only reads; extracted strings go through the same sanitizer. VTE/GUI rendering of untrusted bytes is the widget's job (VTE already handles this).
• Persistence: state_dir 0700; snapshot contains no secrets by construction (env allowlist); scrollback tails may contain credentials the user pasted — file perms 0600, documented, with a redact option to disable tail persistence per workspace.
• Executable plugins (Phase 4): run with full user privileges; install is explicit and the trust requirement is documented at install time. No sandboxing in MVP; investigated later.

Non-goals (MVP)

Multi-user servers, network exposure, privilege separation between server/workers, secret redaction inside scrollback content, audit log (signing). Revisit if a network listener is ever added.