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STARTTLS is vulnerable to man-in-the-middle and encryption downgrade attacks. Using a distinct port that only allows these application protocols over SSL/TLS (e
by jolmg 8y ago
STARTTLS is vulnerable to man-in-the-middle and encryption downgrade attacks. Using a distinct port that only allows these application protocols over SSL/TLS (e.g. SMTPS and IMAPS) is better. It's also widely accepted. I don't think there's many that are still running unencrypted SMTP or IMAP connections.
- jcranmer 8y agoOpportunistic STARTTLS is vulnerable to encryption downgrade attacks. Good email clients don't implement opportunistic STARTTLS, they implement mandatory STARTTLS or TLS, with plaintext being either an option that's heavily discouraged by UX flow, enabled only for tests, or not available at all.
- jolmg 8y agoSure, but a user has no way of knowing if their mail client does opportunistic STARTTLS or not. It's not something the average user can test, and not something mail clients would typically feel the need to mention. On the other hand, you can tell people that good practice is to use ports 993 for IMAP and 465 for SMTP and they'll be good to go, that using any other ports opens them to potential attack.