Behavior: FileZilla's certificate trust persistence for self‑signed FTPS hosts

Answer to the Core Question

FileZilla should not automatically skip hostname verification** for a user‑trusted self‑signed certificate**. The client must still verify that the hostname presented by the server matches the one the user originally trusted, and that the certificate’s fingerprint has not changed. Only an optional, user‑controlled bypass that performs strict hostname‑and‑fingerprint checks should be considered.

Why the Prompt Persists

• FileZilla’s certificate manager stores the certificate but does not record the hostname it was trusted for.
• During each connection the TLS stack performs hostname verification against the server’s CN or SAN fields.
• Because the manager has no host‑record, the client treats every new session as a fresh validation request, even if the same certificate is presented.

Risks of Unconditional Bypass

• Man‑in‑the‑middle attacks: a malicious host could present the same self‑signed certificate under a different domain and be accepted.
• Certificate rotation: a legitimate server may rotate its key; an unconditional bypass would silently accept the new cert, masking the change.

Recommended Safe Approach

1. Store hostname + fingerprint. When a user accepts a certificate, record the hostname and the SHA‑256 fingerprint.
2. On each new connection, compare the presented fingerprint and hostname against the stored values.
3. If either differs, prompt the user again. If both match, the connection can proceed without a prompt.

This preserves the convenience of persistent trust while guarding against host or certificate changes.

Optional Feature Flag

FileZilla could expose a setting such as Allow Trusted Certs to Bypass Hostname Check. When enabled, the above hostname‑and‑fingerprint check is performed automatically. The UI should display a clear warning whenever a mismatch occurs, and the user must confirm acceptance.

Steps to Verify Current Behavior

1. Connect to an FTPS server with a self‑signed cert and accept it.
2. Check FileZilla → Settings → FTP → FTP over TLS/SSL → Certificate Manager to confirm the cert is listed.
3. Close and reopen FileZilla, then reconnect to the same hostname. The prompt should reappear.
4. Change the server’s certificate (keeping the same hostname). Reconnect; the prompt should appear again.

Missing Diagnostic Detail

To tailor the recommendation to your environment, could you confirm whether your organization requires an automated bypass for trusted hosts, or if strict per‑session validation is acceptable?