Hello, On 2021-08-10, at 07:39 (UTC-0700), Stephen Satchell had the following to say:
: On 8/10/21 7:00 AM, Mono DHS wrote: : > Are there plans to revisit the SMTP command parsing and handling logic : > in the server in one form or another? Are people making active use of : > the smtpd_forbidden_commands parameter? : > : : Longer answer: See this shell sequence: : > # postconf smtpd_forbidden_commands : > smtpd_forbidden_commands = CONNECT GET POST : > # postconf mail_version : > mail_version = 3.4.13 Same here (same version), since it's the default. Anyone added more or other tokens, like, e.g. "EXPUNGE" and "RENAME" (both IMAP), "HAVESPACE" and "SETACTIVE" (both ManageSieve) or anything else imaginable that may or may not be thrown at the server at any point in time? An open door philosophy such as the one implied by smtpd_forbidden_commands does and can /not/ provide protection from the big bad world, even and especially in practical terms. This is simply because it starts with an "anything permitted" policy and only then selectively, more or less arbitrarily, closes the door for specific tokens that have been observed in the wild, perhaps as part of a deliberate action or just misconfiguration. Administrators of such a system are relegated to eternally playing catch-up with the outside world. In contrast, robust protocol implementations are designed according to a constructivist philosophy, starting with a clean slate, "nothing permitted" policy and only ever adding what's permitted by the protocol, and that protocol alone. Any input such a server is presented with that lies outside that protocol definition is rejected outright or dealt with in other ways according to local policy. And the right to reject non-SMTP input follows from operating an SMTP MTA on the well-known TCP port 25, while at the same time holding up the fundamental principle upon which the success of the Internet is based, namely of being liberal in what one accepts and conservative in what one sends. Cheers, Mono
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