CVEs / CVE-2025-66376

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

Known exploited in the CISA KEV catalog since 18 Mar 2026
medium 6.1 CVSS 3.1 (NVD)
EPSS 20.2% chance of exploitation in the next 30 days, higher than 97% of CVEs (30 Oct 2026)
Last changed at its sources 28 Oct 2026

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Summary

Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability in the Classic UI where attackers could abuse Cascading Style Sheets (CSS) @import directives in email HTML.
CISA, Known Exploited Vulnerabilities catalog

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.
The CVE record, via NVD · published 5 Jan 2026 · NVD status: Analyzed

Affected and fixed versions

As stated by the CVE record (from the CNA)
Product	Affected	Fixed in
Zimbra Collaboration	from 10.0, before 10.0.18	10.0.18
Zimbra Collaboration	from 10.1, before 10.1.13	10.1.13

Exploitation

In CISA KEV since	18 Mar 2026
Due date for US federal agencies	1 Apr 2026
Used in ransomware campaigns	Unknown
Required action	Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Severity

CVSS 3.1	6.1 medium `CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N` Scored by NVD
Weakness	CWE-79

References

CVE-2025-66376 on NVD
CVE-2025-66376 in the CISA KEV catalog
https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories (cited by CISA)
CVE-2025-66376 on OSV
https://wiki.zimbra.com/wiki/Security_Center Release Notes, Vendor Advisory
https://wiki.zimbra.com/wiki/Zimbra_Releases/10.0.18#Security_Fixes Release Notes
https://wiki.zimbra.com/wiki/Zimbra_Releases/10.1.13#Security_Fixes Release Notes
https://wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_Policy Product
https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories Vendor Advisory
https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-66376 US Government Resource

Sources

Exploitation status from the CISA Known Exploited Vulnerabilities catalog. EPSS scores from FIRST. Descriptions, severity and references from the National Vulnerability Database. Open-source package versions from OSV (CC-BY 4.0). Text is shown as each source states it; Stackurity does not edit it. Check the vendor's advisory before acting.

This product uses data from the NVD API but is not endorsed or certified by the NVD.