CVE-2025-66376
Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability
Does this matter?
Known to be exploited in the wild (CISA KEV, CISA remediation deadline 1 April 2026). Treat as an emergency change: patch or isolate now, then hunt for prior compromise.
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.
- CVSS 3.1
- 6.1 MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
- EPSS
- 20.23% probability · 97th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Listed 18 March 2026 · due 1 April 2026
- Weakness
- CWE-79
- Affected
- synacor/zimbra collaboration suite
- Source
- cve@mitre.org
EPSS moved from 17.3% on 7 October 2026 to 20.2% on 30 October 2026. The dashed line is the ten per cent mark our analysts treat as “patch before the next change window”.
CISA notes
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories ; https://nvd.nist.gov/vuln/detail/CVE-2025-66376
References
- https://wiki.zimbra.com/wiki/Security_CenterRelease Notes, Vendor Advisory
- https://wiki.zimbra.com/wiki/Zimbra_Releases/10.0.18#Security_FixesRelease Notes
- https://wiki.zimbra.com/wiki/Zimbra_Releases/10.1.13#Security_FixesRelease Notes
- https://wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_PolicyProduct
- https://wiki.zimbra.com/wiki/Zimbra_Security_AdvisoriesVendor Advisory
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-66376US Government Resource
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.