Polish comment + grade cap reason for STARTTLS

Author: drwetter

testssl.sh

@@ -22933,20 +22933,18 @@ run_rating() {
      pr_headlineln " Rating (experimental) "
      outln
 
-
-     if [[ -n "$STARTTLS_PROTOCOL" ]]; then
-          read -r -d '' grade_cap_reason <<'EOF'
-TL;DR: E-mail transfer via port 25 is broken and the amendments suggested so far are duct tape. So please do not expect testssl.sh to shut up.
-
-Explanation: For other than SMTP you should use TLS as per RFC 8314. For SMTP however there's this thing named reality: A mail server cannot
-just switch to the mail submission port 587 only and continue to receive mail from everyone. Even if you advertise this via SRV record (RFC 6186).
-For STARTTLS there's no way to tell for testssl.sh whether it is secure. A MitM can always intercept the connection, unless the client checks
-the certificate accordingly (it's getting better but some just don't). TLSA Records/DANE and MTA-STS (RFC-8461) on the server side can help too.
-But as said, it's useless unless the client MTA checks all that which no tool can check.
-EOF
-          # We can't use newlines in the message, as the grade-sorting function will mess up the reason
-          set_grade_cap "T" "$(tr '\n' ' ' <<<$grade_cap_reason)"
-     fi
+     [[ -n "$STARTTLS_PROTOCOL" ]] && set_grade_cap "T" "STARTTLS is prone to MITM downgrade attacks. A secure TLS upgrade can only be ensured client-side. You should use TLS only (=implicit TLS) rather than STARTTLS as per RFC 8314, for other than SMTP and SIEVE"
+
+     # TL;DR: STARTTLS connections are inherently insecure. A MITM can always intercept the connection, unless the client checks e.g. the
+     # certificate accordingly. A secure STARTTLS client is the key but we can't test for it. For other than SMTP and SIEVE (there's no implicit TLS port)
+     # you should use implicit TLS as per RFC 8314. Especially e-mail transfer via port 25 is broken and amendments so far are duct tape.
+
+     # Explanation: There are active MitM attacks possible when using STARTTLS like https://github.com/tintinweb/striptls or
+     # https://github.com/libcrack/starttlsstrip. It depends on the client only whether it can detect such downgrade attack.
+     # As some SMTP servers are still misconfigured with wrong certificates it's is still common practice for SMTP client MTAs to
+     # accept those wrong certificates -- delivering e-mails is more important. There is an e-mail submission port 587 but a mail server
+     # cannot just switch to it and continue to receive mail from everyone. Even if you advertise this via SRV record (RFC 6186).
+     # TLSA Records/DANE and MTA-STS (RFC-8461) on the server side can help too,
 
      pr_bold " Rating specs"; out " (not complete)  "; outln "SSL Labs's 'SSL Server Rating Guide' (version 2009q from 2020-01-30)"
      pr_bold " Specification documentation  "; pr_url "https://github.com/ssllabs/research/wiki/SSL-Server-Rating-Guide"