Disable DH 1024

How to change the Diffie Hellman key size?

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Mailbox Level

Problem

Updating "zimbraSSLDHParam" affects at proxy level only. In case you want steps to configure the proxy and other Internet facing services read: https://wiki.zimbra.com/wiki/Cipher_suites. In below article you find steps on how to update diffie helman (DH) key size at mailbox level.

Solution

By adding "jdk.tls.ephemeralDHKeySize" into the mailboxd_java_options DH key size can be changed (or disable weak DH 1024) at mailbox level.

Notes:

These commands are restricted to run on mailbox servers only.

JDK 17 supports max key size value 2048.

Step 1:

Get existing value of the mailboxd_java_options.

Example:

$ zmlocalconfig mailboxd_java_options
mailboxd_java_options = -server -Dhttps.protocols=TLSv1.2 -Djdk.tls.client.protocols=TLSv1.2 -Djava.awt.headless=true -Dsun.net.inetaddr.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.security.egd=file:/dev/./urandom --add-opens java.base/java.lang=ALL-UNNAMED

Step 2:

Add "-Djdk.tls.ephemeralDHKeySize=2048" to mailboxd_java_options.

Example:

$ zmlocalconfig -e mailboxd_java_options="-server -Dhttps.protocols=TLSv1.2,TLSv1.3 -Djdk.tls.client.protocols=TLSv1.2,TLSv1.3 -Djdk.tls.ephemeralDHKeySize=2048 -Djava.awt.headless=true -Dsun.net.inetaddr.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.security.egd=file:/dev/./urandom --add-opens java.base/java.lang=ALL-UNNAMED"

Step 3:

Restart mailbox services

$ zmmailboxdctl restart

Step 4:

Verify using nmap

nmap --script ssl-enum-ciphers -p 8443 your-mailbox-server.example.com

Ref: https://docs.oracle.com/javase/8/docs/technotes/guides/security/jsse/JSSERefGuide.html#jsse-impl-security-property

Global/Proxy Level

Update DH parameters at proxy level. No longer self generated, use pre-defined DHE groups as recommended by IETF RFC 7919.

Note: the zmdhparam command will in most cases not work when used with OpenSSL FIPS.

Further reading:

• https://weakdh.org/
• https://github.com/internetstandards/dhe_groups

wget https://raw.githubusercontent.com/internetstandards/dhe_groups/master/ffdhe4096.pem -O /etc/ffdhe4096.pem
su - zimbra
zmprov mcf zimbraSSLDHParam /etc/ffdhe4096.pem
ldap restart
zmmtactl restart
zmproxyctl restart

Submitted by: Raghu Noti