Disabling SMTP-SSL/465 SSLv3 on Zimbra 8.6.0

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Disabling SMTP-SSL/465 SSLv3 on Zimbra 8.6.0

Postby kbulgrien » Thu Nov 08, 2018 11:43 pm

While considering an upgrade from Zimbra 8.6.0 P12 to 8.7 or later, it came to light that even though we thought that SSLv3 was already disabled on this server, that it is not fully disabled despite Zimbra configuration that seems as though it should be.


The configuration at the time did attempt to disable SSLv3, but apparently this was insufficient to fully accomplish it.

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$ zmprov gacf | grep -i SSLv3
zimbraMtaLmtpTlsProtocols: !SSLv2, !SSLv3
zimbraMtaSmtpTlsProtocols: !SSLv2, !SSLv3
zimbraMtaSmtpdTlsProtocols: !SSLv2, !SSLv3

In particular, it was noted that the test at the bottom of https://wiki.zimbra.com/wiki/How_to_disable_SSLv3#Testing_SMTP-SSL showed that SMTP-SSL on port 465 was still active

https://www.htbridge.com/ssl/ confirmed that indeed the server was using SSLv3.

https://wiki.zimbra.com/wiki/Security/Collab/86#MTA alerted me to the fact that Zimbra 8.6.0 does not disable SSLv3 completely due to a bug (https://bugzilla.zimbra.com/show_bug.cgi?id=97186).

Though both the aforementioned documents alluded to Postfix parameters that needed to change to disable SSLv3, details were sparse. I eventually found a StackOverflow answer that explain more clearly (https://serverfault.com/a/670347/101931).

I was able to disable SSLv3 more completely by editing /opt/zimbra/postfix/conf/main.cf and by appending:

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smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3
lmtp_tls_mandatory_protocols = !SSLv2, !SSLv3

The manual changes seemed to persist after a server restart:

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$ zmmtactl restart
Rewriting configuration files...done.
Stopping saslauthd...done.
Starting saslauthd...done.
/postfix-script: refreshing the Postfix mail system

They also seemed to persist following zmcontrol restart.

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$ tail -3 ~/postfix/conf/main.cf
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3
lmtp_tls_mandatory_protocols = !SSLv2, !SSLv3

It seems that this works as the htbridge test report shows a change for the better, and the manual edit of main.cf seems to persist through rewrites by Zimbra configuration tools.

Is a better or more appropriate way to have accomplished this change in 8.6.0?

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