CVE-2025-66376: Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability

Severity: medium (CVSS 6.1 v3.1)
Exploited in the wild: yes — in CISA KEV since 2026-03-18
EPSS: 20.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-01-05 · updated 2026-10-07
Weakness: CWE-79

Description

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.

Affected products

Zimbra Collaboration: from 10.0.0 before 10.0.18; from 10.1.0 before 10.1.13 — fixed in 10.0.18, 10.1.13

How to fix

Upgrade to a fixed version: Zimbra Collaboration 10.0.18, 10.1.13. Check the vendor advisory for mitigations.

References

https://wiki.zimbra.com/wiki/Security_Center
https://wiki.zimbra.com/wiki/Zimbra_Releases/10.0.18#Security_Fixes
https://wiki.zimbra.com/wiki/Zimbra_Releases/10.1.13#Security_Fixes
https://wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_Policy
https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-66376

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