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. Configuration settings on this page are routinely validated by our QA team.
If you installed or upgraded to Zimbra version 9.0.0.P34, 8.8.15.P41, 10.0.2 or higher, Zimbra will use OpenSSL 3.0.x and FIPS compliance for OpenSSL will be enabled by default. Using FIPS mode for OpenSSL is more secure as weak ciphers will not be available and many security issues do not affect OpenSSL FIPS.
To find out if Zimbra OpenSSL is FIPS enabled, you can run the following command that should fail with
Error setting digest
:
<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
The following headers will:
zmprov mcf +zimbraResponseHeader "Strict-Transport-Security: max-age=31536000; includeSubDomains"
zmprov mcf +zimbraResponseHeader "X-Content-Type-Options: nosniff"
zmprov mcf +zimbraResponseHeader "X-Robots-Tag: noindex"
zmprov mcf +zimbraResponseHeader "Referrer-Policy: no-referrer"
zmprov mcf zimbraMailKeepOutWebCrawlers TRUE
zmmailboxdctl restart
wget https://raw.githubusercontent.com/internetstandards/dhe_groups/master/ffdhe4096.pem -O /etc/ffdhe4096.pem
su - zimbra
zmprov mcf zimbraSSLDHParam /etc/ffdhe4096.pem
Reboot the server.
Postfix traffic is not routed through Zimbra proxy. Below commands show how to configure Zimbra MTA to use only strong TLS ciphers. In 2023 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
nmap --script ssl-enum-ciphers -p 465 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
Configure Zimbra MTA Postfix 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=$(/opt/zimbra/common/bin/openssl ciphers)
postconf -e tls_preempt_cipherlist=no
zmprov gs `zmhostname` zimbraMtaTlsAuthOnly
zmprov ms `zmhostname` zimbraMtaTlsAuthOnly TRUE # if not already (this is default)
zmmtactl restart
Run again to verify your set-up:
nmap --script ssl-enum-ciphers -p 25 your-mta-server.example.com
nmap --script ssl-enum-ciphers -p 465 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.
Check and see if TLSv1.0 and TLSv1.1 are enabled (default) and what the least strength cipher is for TLSv1.2 and above (default: A).
To force the use of TLS >= v1.2 with strong Ciphers run the following:
zmlocalconfig -e ldap_common_tlsprotocolmin="3.3"
zmlocalconfig -e ldap_common_tlsciphersuite="HIGH"
In addition require TLS for LDAP (disable unencrypted LDAP) via:
zmlocalconfig -e ldap_starttls_supported=1
zmlocalconfig -e zimbra_require_interprocess_security=1
zmlocalconfig -e ldap_starttls_required=true
For this change it is recommended to restart Zimbra using zmcontrol restart.
With the above setting the Zimbra POP3 implementation requires the client to issue the STLS command. This command will switch from cleartext to encrypted communications.
If the STLS command is not issued, any command the client sends such as AUTH or USER to Zimbra will result in an error and the client will not try authentication. This means the password is not send without encryption. In addition email contents and attachments are also transmitted using encrypted communication.
False positives in OpenVAS and warnings in email clients such as Thunderbird
Email clients and vulnerability scanner can send some commands in plain text to Zimbra, such as CAPA (to list capabilities) and Zimbra will respond to these without encryption. This will make vulnerability scanners such as OpenVAS believe POP3 is enabled for unencrypted connections. This is however not the case. The false positive will look like this:
The remote host is running a POP3 daemon that allows cleartext logins over unencrypted connections.
For the same reason you can add your Zimbra account with POP3 to Thunderbird (and other clients) and select Connection security: none this will trigger a warning, saying your credentials will be transmitted without encryption. In reality the communication between the client and Zimbra will halt because of errors before authentication unless TLS is used.
This has been verified by using Wireshark.
It is not recommended to expose the Admin UI to the Internet. Instead administrators should access Admin UI via a VPN. In any case you will need to make sure to proxy the Admin UI via Zimbra Proxy to make sure it uses the best TLS configuration. This means you should access Admin UI via the proxied port 9071, and deny access to port 7071 via a firewall. To enable this you should run as user Zimbra:
/opt/zimbra/libexec/zmproxyconfig -e -w -C -H `zmhostname`
zmproxyctl restart
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.
All information contained in this blog is intended for informational purposes only. Synacor, Inc. is not responsible or liable in any manner for the use or misuse of any technical content provided herein. No specific or implied warranty is provided in association with the information or application of the information provided herein, including, but not limited to, use, misuse or distribution of such information by any user. The user assumes any and all risk pertaining to the use or distribution in any form of any subject matter contained in this blog.
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