This is announcement published by Synacor regarding the zero-day exploit vulnerability for Log4j
After intensive review and testing, Zimbra Development determined that the zero-day exploit vulnerability for Log4j (CVE-2021-44228) does not affect the current supported Zimbra versions 9.0.0 and 8.8.15.
Zimbra Collaboration Server currently uses Log4j version 1.2.16. The cause of the vulnerability is found in the lookup expression feature in Log4j versions 2.0 to 2.17.
Also, the Redhat (CVE-2021-4104) vulnerability does not affect the current Supported Zimbra Collaboration Server versions 9.0.0 and 8.8.15. For this vulnerability to affect the server, it needs JMSAppender and the ability to append configuration files. Zimbra does not use the JMSAppender.
The Zimbra Development team is in the process of upgrading Log4j which is expected to be completed within Q1 2022.