Am 06.12.2014 um 14:40 schrieb Daniel Parthey:
Am 6. Dezember 2014 13:10:58 MEZ, schrieb Reindl Harald 
<h.rei...@thelounge.net>:

Am 06.12.2014 um 06:56 schrieb Jan Wideł:
If you add disable_plaintext_auth=yes ssl=required settings, then
dovecot will drop authentication without STARTTLS. But damage will be
done, client will send unencrypted (or in this scenario MD5 or SHA512
hash) login/password

no, damage will *not* be done

STARTTLS happens in context of connect and *log before* any
authentication is tried the handshake between client/server fails

If the client is misconfigured to not strictly require STARTTLS, but to allow 
plaintext authentication too, and some man in the middle strips the STARTTLS 
capability from the server capability message, then the client will probably 
send its password login attempt in plaintext, without even trying to establish 
a STARTTLS session, because the server seemed to be incapable of STARTTLS.

So you might need to teach your users to enforce STARTTLS in their email client 
in order to mitigate MITM attacks

that's so far true but:

* if you require STARTTLS try to setup the account without TLS
  fails while "not strictly require STARTTLS" is a issue of the
  past where Thunderbird offered "TLS if available"

* so that MITM needs to happen in the timeframe where the user
  configures the account the first time not chose STARTTLS

* after the account was configured that MITM is no longer possible

MITM which strips STARTTLS is more an issue in case of opportunistic TLS between MTA's

frankly i still don't understand the stupidity of deprecate 465 in favor of STARTTLS instead use 465/993/995 in context of mail-clients which would "MITM strip away STARTTLS" not make possible at all

from a straight technical point of view the only *real* use-case for STARTTLS ist MTA-to-MTA on Port 25 which don't send credentials at all



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