Cipher suites

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Cipher suites

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Enable Strong Ciphers

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:

ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384;

Configuring Zimbra Proxy Nginx

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

zmprov -l mcf zimbraReverseProxySSLCiphers 'ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384'

zmproxyctl restart

Configuring Zimbra Mailbox

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>
</key>

Then restart mailbox, or reboot your server:

zmmailboxdctl restart

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

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.

Configuring Zimbra MTA Postfix

Postix traffic is not routed through Zimbra proxy. Below commands show how to configure Zimbra MTA to use only strong TLS ciphers. In 2021 not all mail servers on the Internet support encryption. For maximum compatibility it is still recommended to use Opportunistic TLS. So that you can receive email via unencrypted transmissions. However you can set zimbraMtaTlsSecurityLevel to encrypt to force the use of TLS. This will result in mail delivery issues.

To test the current state of the MTA run from the MTA:

nmap --script ssl-enum-ciphers -p 25 your-mta-server.example.com

The last line of output with Zimbra default config: least strength: F

openssl s_client -starttls smtp -showcerts -connect your-mta-server.example.com:25 -servername your-mta-server.example.com -tls1_1

Then the following configuration will remove weak ciphers and disable some Postfix options that are considered unsecure.

Find the current list of ciphers for Postfix via:

https://ssl-config.mozilla.org/#server=postfix

Configure it in Zimbra using:

zmprov mcf zimbraMtaSmtpdTlsCiphers medium
zmprov mcf zimbraMtaSmtpdTlsMandatoryCiphers  medium
zmprov mcf zimbraMtaSmtpdTlsProtocols '>=TLSv1.2'
zmprov mcf zimbraMtaTlsSecurityLevel may
postconf -e fast_flush_domains=""
postconf -e smtpd_etrn_restrictions=reject
postconf -e disable_vrfy_command=yes
postconf -e tls_medium_cipherlist="ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384"
postconf -e tls_preempt_cipherlist=no
zmmtactl restart

IT IS VERY IMPORTANT tls_medium_cipherlist IS SET, setting just medium or high in zimbraMtaSmtpdTlsCiphers/zimbraMtaSmtpdTlsMandatoryCiphers will not work!!

Above config was tested with email from Gmail (uses tls), Ubuntu 20 Postfix (uses tls), from Zimbra itself (uses lmtp) and http://ismyemailworking.com/ (uses plain text) and this all works.

Run again to verify your set-up:

nmap --script ssl-enum-ciphers -p 25 your-mta-server.example.com

The last line of output with Zimbra new config: least strength: A

It seems TLS v1.3 is either not enabled or not tested via nmap, but you can verify that like so:

openssl s_client -starttls smtp -showcerts -connect your-mta-server.example.com:25 -servername your-mta-server.example.com -tls1_3

openssl s_client -starttls smtp -showcerts -connect your-mta-server.example.com:25 -servername your-mta-server.example.com -tls1_1

Please note that you can best run nmap/openssl commands on your MTA server to avoid firewall and network blocking issues of port 25.

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.
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