FileZilla 3.60.0: Explicit TLS enforcement now mandatory – how to support legacy FTP servers?

Resolution

To support legacy servers without modifying every Site Manager entry, enable the global "Fallback to plain FTP on TLS failure" setting. This allows connections to proceed as plain FTP when the TLS handshake fails, even if the specific site is configured to "Require explicit FTP over TLS."

Technical Behavior and Logic

In FileZilla 3.60.0, the enforcement of explicit TLS changes how the client handles handshake failures. The behavior differs based on where the configuration is applied:

Global Fallback vs. Site Manager Settings

• Global Fallback Enabled: The client attempts an explicit TLS handshake first. If the server does not support TLS or the handshake fails, the client automatically retries the connection using plain FTP. This overrides the "Require explicit FTP over TLS" restriction in the Site Manager for that session.
• Site Manager "Use explicit FTP if available": This tells the client to negotiate TLS if the server advertises it. However, if the server does not advertise TLS and the global fallback is disabled, the connection will fail.

Impact on Automation and Legacy Protocols

Automated scripts and legacy workflows are affected as follows:

• Plain FTP: Scripts will continue to function provided the global fallback is enabled or the site is explicitly set to plain FTP.
• Implicit TLS: FileZilla 3.60.0 has deprecated support for implicit TLS. Scripts relying on this must be migrated to explicit TLS or plain FTP.
• Failure Detection: Scripts that previously relied on a hard connection failure to detect the absence of TLS will now succeed (via fallback) unless the global setting is disabled.

Verification Steps

To verify this behavior in a test environment, use the following sequence:

1. Navigate to Settings > FTP > Explicit FTP over TLS and enable "Fallback to plain FTP on TLS failure."
2. Configure a Site Manager entry for a non-TLS server set to "Require explicit FTP over TLS." Attempt a connection; it should succeed after a failed handshake.
3. Configure a Site Manager entry for a TLS-enabled server. Verify that the connection remains encrypted (check the status bar/logs) and does not fall back to plain FTP.

Security Assumption

This configuration assumes the test environment is isolated. Enabling global fallback reduces security by allowing unencrypted traffic when TLS negotiation fails, which could expose credentials to man-in-the-middle attacks if used in production.