On Tue, 29 Oct 2024, Brian E Carpenter wrote:
The Security Considerations start by saying "SMTP mail is inherently insecure" which is undoubtedly true, although "Transmission of mail via SMTP is inherently insecure" might be more precise. So I am a bit surprised that the next sentence doesn't require STARTTLS and cite RFC 3207 and RFC 7817.
I would have hoped we would agree that we do not want to make breaking changes to a widely deployed 40 year old protocol. While it is true that most mail over the public Internet uses STARTTLS, you and I do not know all of the places SMTP is used, it's always been an optional feature, and we at least used to give lip service to maintaining interoperability.
As I said before I personally would not be opposed to mentioning somewhere that SMTP, like most of our other application protocols, is subject to observation for which STARTTLS can help (although as Dave noted, PGP and S/MIME address the issue at a different level), and it is subject to MITM attacks, also like most of our other application protocols, for which MTA-STS and TLSA can help. But they are all optional extensions, not part of SMTP, which is why I don't want them cluttering up the SMTP spec.
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