Fold a conservative security-header set into the SAME opt-in proxy mode, so it's
zero-impact when off. When security.trust_reverse_proxy is on, an after_request
adds X-Content-Type-Options: nosniff, X-Frame-Options: SAMEORIGIN, and HSTS
(safe — only honoured over the proxy's HTTPS), via setdefault so it never clobbers
a header the proxy already set. No CSP (needs per-deploy tuning; better at the
proxy). When OFF (default), the after_request isn't registered → no headers added.
Tests: off adds none of the headers; on adds all three. Doc updated. 6 tests pass.
Tier 1 of "secure behind a reverse proxy". STRICTLY opt-in so direct/LAN installs
are byte-for-byte unchanged.
- core/security/reverse_proxy.py: apply_reverse_proxy_mode(app, config_get) — a
no-op unless security.trust_reverse_proxy=true. When OFF (default), the app is
untouched: no ProxyFix, X-Forwarded-* stays UNtrusted (a direct client can't
spoof its IP/scheme), session cookie keeps Flask defaults. When ON (operator is
behind nginx/Caddy/Traefik with TLS): trust one proxy hop's X-Forwarded-*, and
mark the session cookie Secure + SameSite=Lax. Any config error → safe no-op,
never breaks startup.
- Wired once at app init.
- Support/REVERSE-PROXY.md: nginx (with the Socket.IO Upgrade headers people
always miss) / Caddy / Traefik configs, the setting, and the "put auth in front
(Authelia/Authentik/oauth2-proxy)" recommendation + the off-for-plain-HTTP note.
Tests: off (and missing-key, and a config exception) is a strict no-op — not
ProxyFix-wrapped, cookie defaults intact; on wraps ProxyFix + secures the cookie;
and the real web_server app is NOT in proxy mode by default. 5 tests pass.
