Revision as of 14:54, 5 February 2016 by Plobbes (talk | contribs)

Best-Practice Recommendations for a Secure Zimbra Configuration

   KB 21032        Last updated on 2016-02-5  

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The following recommendations are provided to ensure a best-practice security configuration. This includes the following:

  • Run services only on secure channels
  • Require encrypted logins
  • Requiring secure interprocess communications
  • Use end-to-end encryption with S/MIME

SSL Certificates

1. Be sure to have properly configure SSL Certificates

References: see these pages:

2. If possible, use only Commercial CA-Signed Certs


Most secure is to only allow secure methods of accessing the system; however, be careful in making these changes, as all processes need to be configured to connect only to upstream encrypted listeners.

1. Configure the proxy to offer only encrypted HTTPS protocols (run this on every proxy):

zmprov gs `zmhostname` zimbraReverseProxyMailMode
zmprov ms `zmhostname` zimbraReverseProxyMailMode https

Note: it is possible to use "redirect" here, which will redirect HTTP requests to HTTPS using a "302 Redirect" response code. Please note, however, that some clients (such as ActiveSync devices) will not honor a 302 Redirect, and may try to pass unencrypted login information. It is recommended to use only https here for highest security.

2. Configure the mailstore to offer only encrypted HTTPS procotol:

zmprov gs `zmhostname` zimbraMailMode
zmprov ms `zmhostname` zimbraMailMode https

3. Require Proxy to connect to upstream via SSL

zmprov gs `zmhostname` zimbraReverseProxySSLToUpstreamEnabled
zmprov ms `zmhostname` zimbraReverseProxySSLToUpstreamEnabled TRUE

Encrypted Logins


zmprov gs `zmhostname` zimbraMailClearTextPasswordEnabled
zmprov ms `zmhostname` zimbraMailClearTextPasswordEnabled FALSE


zmprov gs `zmhostname` zimbraImapCleartextLoginEnabled
zmprov ms `zmhostname` zimbraImapCleartextLoginEnabled FALSE
zmprov gs `zmhostname` zimbraReverseProxyImapStartTlsMode
zimbraReverseProxyImapStartTlsMode: only


zmprov gs `zmhostname` zimbraPop3CleartextLoginEnabled
zmprov ms `zmhostname` zimbraPop3CleartextLoginEnabled FALSE
zmprov gs `zmhostname` zimbraReverseProxyPop3StartTlsMode
zimbraReverseProxyPop3StartTlsMode: only

Secure Interprocess Communication

1. Make sure LDAP is supporting STARTTLS - should be set to "1":

zmlocalconfig ldap_starttls_supported
zmlocalconfig -e ldap_starttls_supported=1

2. Require interprocess security - should be set to 1:

zmlocalconfig zimbra_require_interprocess_security
zmlocalconfig -e zimbra_require_interprocess_security=1

3. Require secure LDAP from mailboxd - should be set to "true":

zmlocalconfig ldap_starttls_required
zmlocalconfig -e ldap_starttls_required=true

References: see also TLS/STARTTLS_Localconfig_Values


1. Disallow plaintext SMTP AUTH logins

zmprov gs `zmhostname` zimbraMtaTlsAuthOnly

If not already, set to "TRUE":

zmprov ms `zmhostname` zimbraMtaTlsAuthOnly TRUE

2. Use opportunistic outbound STARTLS

zmlocalconfig postfix_smtp_tls_security_level

If not already, set to use "may". Note that it is not usually possible to use "encrypt" here, as you cannot require remote MTAs to use encryption:

 zmlocalconfig -e postfix_smtp_tls_security_level=may

3. Use opportunistic inbound STARTTLS

zmprov gs `zmhostname` zimbraMtaTlsSecurityLevel

If not already, set to use "may". Note that it is not usually possible to use "encrypt" here, as you cannot require remote MTAs to use encryption:

 zmprov ms `zmhostname` zimbraMtaTlsSecurityLevel may

4. Restart the postfix MTA

 zmmtactl restart

End-to-End Encryption

1. Use S/MIME

Reference: see SMIME_Certificates

2. As an Open Source alternative, you can use as well the OpenPGP Zimlet, by Barry de Graaff

Zimbra OpenPGP Zimlet


1. Configure your firewall to allow *only* required ports:

Reference: see Ports

2. Restrict external ssh to use two-factor authentication.

The Google Authenticator is one option for this:

Verified Against: ZCS 8.0 Date Created: 06/28/2014
Article ID: Date Modified: 2016-02-05

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