CVE-2025-66376
Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability
Synacor Zimbra Collaboration Suite (ZCS)
Zimbra Collaboration (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 allows Classic UI stored XSS via Cascading Style Sheets (CSS) @import directives in an HTML e-mail message.
Exploitation status
CISA Known Exploited Vulnerabilities
Listed by CISA on 18 March 2026. US federal agencies were required to remediate it by 1 April 2026.
EPSS — exploitation probability
FIRST puts the probability of exploitation activity in the next 30 days at 20.2%, which is higher than 97.5% 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:N/S:C/C:L/I:L/A:N
Frequently asked questions
Is CVE-2025-66376 being exploited by ransomware groups?
No ransomware group in this database is currently recorded as exploiting CVE-2025-66376.
What does CVE-2025-66376 affect?
CVE-2025-66376 affects Synacor Zimbra Collaboration Suite (ZCS). Zimbra Collaboration (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 allows Classic UI stored XSS via Cascading Style Sheets (CSS) @import directives in an HTML e-mail message.
How severe is CVE-2025-66376?
CVE-2025-66376 is rated HIGH with a CVSS base score of 7.2. EPSS puts the probability of exploitation in the next 30 days at 20.2%, higher than 97.5% of all scored vulnerabilities. It is listed in the CISA Known Exploited Vulnerabilities catalog, with a federal remediation deadline of 1 April 2026.
How should organisations respond to CVE-2025-66376?
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.