proxyseerr is configured with the admin API keys for up to four Sonarr/Radarr instances, and it attaches them to every request it forwards. Anything that can reach its ports can therefore do anything those API keys can do — add and delete media, read your library, change server settings. Treat the proxy's ports with the same care as the servers behind it.

• Authentication is required by default. The proxy refuses to start unless PROXY_API_KEY is set (minimum 16 characters, compared in constant time) or PROXY_ALLOW_ANONYMOUS=true is set deliberately. Requests without the key get 401 before anything is forwarded.
• The upstream API keys never leave the container. Whatever key a caller presents is discarded and replaced with the target instance's own key.
• Request headers are allowlisted. Only Content-Type, Accept and User-Agent are relayed; cookies, Authorization, and forwarding headers are dropped rather than passed to a server that trusts this proxy.
• Response headers are allowlisted too. No Set-Cookie, auth challenges, or upstream server banners are relayed back.
• The forwarded path surface is narrow. Only /api/... and /ping are forwarded. Traversal segments, backslashes and control characters are refused without contacting the upstream.
• Redirects are not followed, so an upstream cannot steer the proxy at a third-party host.
• Errors do not leak topology. Internal URLs and upstream exception text go to the log; callers get a generic message. /proxy/health reports only reachability unless the caller is authenticated.
• Credentials are redacted from logs, including any apikey= in a URL that appears inside an upstream exception message.
• Logs record every rejection: unauthenticated requests, refused paths and oversized bodies each produce a WARNING naming the method and path.
• Request bodies are capped at MAX_BODY_BYTES (8 MB default).
• The container runs as a non-root user (uid 1000).

• Keep the proxy on your LAN. It is not built to face the internet; do not port forward it. If you must expose it, put it behind a reverse proxy that does TLS and its own authentication.
• Talk to it over your trusted network. Traffic between Seerr and the proxy is plain HTTP and carries PROXY_API_KEY.
• Give each Sonarr/Radarr instance its own API key so one can be rotated alone.
• Never commit a real .env, docker-compose.override.yml, or an Unraid template with your keys filled in. All three are gitignored.

Open a security advisory at https://github.com/RonFBurgundy/proxyseerr/security/advisories/new rather than a public issue.