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Using Zimbra with strong TLS configuration

Transport Layer Security (TLS) encrypts data sent over the Internet to ensure that eavesdroppers and hackers are unable to see what you transmit which is particularly useful for private and sensitive information such as passwords, credit card numbers, and personal correspondence. (further reading: https://www.internetsociety.org/deploy360/tls/basics)

In this article you will learn how to configure Zimbra to use only strong encryption ciphers for TLS.

Generate ssl_ciphers for use with zimbraReverseProxySSLCiphers

Since encryption is always evolving it is recommended to use Mozilla SSL Config generator that you can find at https://ssl-config.mozilla.org/

Select Intermediate and Nginx (Zimbra proxy is based on Nginx) at the time of writing this article this will select nginx 1.17.7 and OpenSSL 1.1.1d. The tool also reports the oldest supported clients that work with this configuration: Firefox 27, Android 4.4.2, Chrome 31, Edge, IE 11 on Windows 7, Java 8u31, OpenSSL 1.0.1, Opera 20, and Safari 9.

From the generated config file copy the value from ssl_ciphers:


Configuring Zimbra

Configure Zimbra to use the above ciphers, and enable TLSv1.2 and TLSv1.3 like this:

zmprov mcf zimbraReverseProxySSLProtocols TLSv1.2
zmprov mcf +zimbraReverseProxySSLProtocols TLSv1.3


zmproxyctl restart
Also configure Zimbra mailbox to allow the use of TLSv1.3. Open in a text editor /opt/zimbra/conf/localconfig.xml find the line mailboxd_java_options and set TLSv1.2,TLSv1.3 in https.protocols and jdk.tls.client.protocols. Example result:

<key name="mailboxd_java_options">
  <value>-server -Dhttps.protocols=TLSv1.2,TLSv1.3 -Djdk.tls.client.protocols=TLSv1.2,TLSv1.3 -Djava.awt.headless=true -Dsun.net.inetaddr.ttl=${networkaddress_cache_ttl} -Dorg.apache.jasper.compiler.disablejsr199=true -XX:+UseG1GC -XX:SoftRefLRUPolicyMSPerMB=1 -XX:+UnlockExperimentalVMOptions -XX:G1NewSizePercent=15 -XX:G1MaxNewSizePercent=45 -XX:-OmitStackTraceInFastThrow -verbose:gc -Xlog:gc*=info,safepoint=info:file=/opt/zimbra/log/gc.log:time:filecount=20,filesize=10m -Djava.net.preferIPv4Stack=true</value>
Then restart mailbox, or reboot your server:

zmmailboxdctl restart

Generate DH parameters

Generating DH parameter improves key exchange and mitigates against Logjam attack. Run as Zimbra user. Further reading: https://weakdh.org/

su - zimbra
/opt/zimbra/common/bin/openssl dhparam -out /opt/zimbra/conf/dhparam.pem.zcs 3072
zmprov mcf zimbraSSLDHParam /opt/zimbra/conf/dhparam.pem.zcs
Reboot the server.

Configure additional HTTP headers

The following headers will:

  • Enable HTTP Strict Transport Security (HSTS)
  • Disable search indexing of your server by Google et al.
zmprov mcf +zimbraResponseHeader "Strict-Transport-Security: max-age=31536000; includeSubDomains"
zmprov mcf +zimbraResponseHeader "X-XSS-Protection: 1; mode=block"
zmprov mcf +zimbraResponseHeader "X-Content-Type-Options: nosniff"
zmprov mcf +zimbraResponseHeader "X-Robots-Tag: noindex"
zmprov mcf zimbraMailKeepOutWebCrawlers TRUE
zmmailboxdctl restart

Validate your settings online using SSL Labs

Go to https://www.ssllabs.com/ssltest/analyze.html and enter the the domain name of your Zimbra server. If you followed the steps in this article you should receive an A+ score and there should be no mention of weak ciphers in the report. This article was written in September 2021. In the report take a look at the client devices listed under Handshake Simulation these will give you an idea of the devices your users can use to connect to your Zimbra server. Also validate there are no weak ciphers listed under Cipher Suites.

Zimbra LifeSize Zimlet

  • This Zimlet for Zimbra 8.8.x allows the reservation of Lifesize virtual rooms with integration of login credentials directly in the text of the meeting message.
  • It was made based on code from Barry de Graaff https://github.com/Zimbra-Community/startmeeting.
Zimlet reservation


Reply by Filter Zimlet

With this Zimlet installed you can enable automated replies based on a filter from within your Zimbra Web Mail.

While you could always configure Zimbra filters to send automated replies over CLI using:
/opt/zimbra/bin/zmmailbox -z -m account@domain.com afrl "Filter_name" 
active any address "to,cc" all is "alias@domain.com" reply "Auto reply message" stop
This feature was not available to the end user in the Web Interface. This Zimlet adds a button in Preferences -> Filters -> Create Filter to add this functionality. https://github.com/Zimbra-Community/reply-by-filter

2019 Crowdfunding poll

We have completed the Survey Monkey and the winner is Zimbra FOSS Two Factor authentication.

Based on the survey our initial focus will be Zimbra FOSS 2FA and that will probably be based on PrivacyIDEA and PrivacyIDEA LDAP proxy and will probably be made available via dockerhub.

Spoofing and Phishing Alert

In this post I like to introduce you to the Spoofing and Phishing Alert Zimlet (previously sa-alert). The goal of this Zimlet is to help users to identify spoofing and phishing and thus offer protection against it. While some parts of this Zimlet work automatically, it is not meant to be a fully automated plug-and-play solution and it is recommended you deploy it with a knowledgeable helpdesk/support staff to back it up.

This zimlet checks the result from Spam Assassin and alerts the user when certain tags are found. In addition it enforces the zimbraPrefShortEmailAddress setting to be FALSE as that allows the user to see the used email FROM address. The Zimlet also checks for suspicious characters in headers, like the NULL character etc. See also Mailsploit and bug 108709.

I deployed the Zimlet in an organisation with 700 users, and pointed the alertmail property to the helpdesk ticketing system, after a few weeks of increased incoming tickets and configuring the ignorelistReplyTo and ignorelistReturnPath the number of false positives dropped, and now the alert is really valuable to the user.