An issue was discovered in the Calendar feature in Zimbra Collaboration Suite 8.8.x before 8.8.15 patch 30 (update 1), as exploited in the wild starting in December 2021. An attacker could place HTML containing executable JavaScript inside element attributes. This markup becomes unescaped, causing arbitrary markup to be injected into the document.
https://www.tenable.com/blog/aa23-215a-2022s-top-routinely-exploited-vulnerabilities
https://www.cisa.gov/news-events/cybersecurity-advisories/aa23-215a
https://thehackernews.com/2023/03/from-ransomware-to-cyber-espionage-55.html
https://www.mandiant.com/resources/blog/zero-days-exploited-2022
https://www.tenable.com/cyber-exposure/tenable-2022-threat-landscape-report
https://www.cisa.gov/news-events/cybersecurity-advisories/aa22-228a
https://wiki.zimbra.com/wiki/Zimbra_Releases/8.8.15/P30
https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories