On 12/10/22 16:48, Alex wrote:
> Hi,
> On Thu, Dec 8, 2022 at 2:17 AM Matus UHLAR - fantomas <uh...@fantomas.sk>
> wrote:
>> On 07.12.22 12:28, Alex wrote:
>>> smtp_tls_security_level = may
>>> smtpd_tls_security_level = may
>>> smtp_tls_mandatory_protocols    = !SSLv2,!SSLv3,!TLSv1,!TLSv1.1
>>> smtp_tls_protocols              = !SSLv2,!SSLv3,!TLSv1,!TLSv1.1
>> so, you don't enforce TLS on a server-server communication (correct), but
>> you disable tlsv1 and tlsv1.1 protocols.
>> This means, if you communicate with older server supporting up to TLS 1.1
>> or
>> 1.0, communication will be unencrypted.
>> This does not make much sense - tls1.0 is better than plaintext.
> I think I assumed there was a vulnerability, like there is with SSLv3, that
> lead me to disable it.
> I've now changed it to just:
> smtpd_tls_mandatory_protocols   = >=TLSv1.0
> Can I also ask if it's a security risk from an information disclosure
> perspective to have multiple domains on the same letsencrypt cert? Each
> postfix instance I have configured processes mail for a number of different
> domains, so it's possible a user could ascertain info about those other
> clients by querying the cert directly. It certainly makes it easier for me
> to maintain the certs, but wanted to consider at what cost to privacy or
> the disclosure of that info.

The most important part is not supporting RSA key exchange.  If you support
RSA key exchange you may be vulnerable to e.g. ROBOT or Bleichenbacher’s CAT.
Demi Marie Obenour (she/her/hers)

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