CVE-2024-27443
Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Vulnerability
Synacor Zimbra Collaboration Suite (ZCS)
An issue was discovered in Zimbra Collaboration (ZCS) 9.0 and 10.0. A Cross-Site Scripting (XSS) vulnerability exists in the CalendarInvite feature of the Zimbra webmail classic user interface, because of improper input validation in the handling of the calendar header. An attacker can exploit this via an email message containing a crafted calendar header with an embedded XSS payload. When a victim views this message in the Zimbra webmail classic interface, the payload is executed in the context of the victim's session, potentially leading to execution of arbitrary JavaScript code.
Exploitation status
CISA Known Exploited Vulnerabilities
Listed by CISA on 19 May 2025. US federal agencies were required to remediate it by 9 June 2025.
EPSS — exploitation probability
FIRST puts the probability of exploitation activity in the next 30 days at 23.6%, which is higher than 97.8% of all scored vulnerabilities. EPSS is a forecast of activity, not a measure of severity, and it is rescored daily.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Frequently asked questions
Is CVE-2024-27443 being exploited by ransomware groups?
No ransomware group in this database is currently recorded as exploiting CVE-2024-27443.
What does CVE-2024-27443 affect?
CVE-2024-27443 affects Synacor Zimbra Collaboration Suite (ZCS). An issue was discovered in Zimbra Collaboration (ZCS) 9.0 and 10.0. A Cross-Site Scripting (XSS) vulnerability exists in the CalendarInvite feature of the Zimbra webmail classic user interface, because of improper input validation in the handling of the calendar header. An attacker can exploit this via an email message containing a crafted calendar header with an embedded XSS payload. When a victim views this message in the Zimbra webmail classic interface, the payload is executed in the context of the victim's session, potentially leading to execution of arbitrary JavaScript code.
How severe is CVE-2024-27443?
CVE-2024-27443 is rated MEDIUM with a CVSS base score of 6.1. EPSS puts the probability of exploitation in the next 30 days at 23.6%, higher than 97.8% of all scored vulnerabilities. It is listed in the CISA Known Exploited Vulnerabilities catalog, with a federal remediation deadline of 9 June 2025.
How should organisations respond to CVE-2024-27443?
Apply the vendor patch for Synacor Zimbra Collaboration Suite (ZCS) as the first priority, and treat any internet-facing instance as the most urgent. Because this vulnerability is associated with ransomware activity, also review whether the affected systems were reachable before patching, rather than assuming that patching alone closes the incident.