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

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.

01What is this vulnerability?

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.

02Affected products

Vendor	Product	Versions
Zimbra	Collaboration	10.0, 10.1

03Active exploitation status

Yes — actively exploited. Added to the CISA KEV catalog on 2026-03-18. Ransomware use: Unknown.

04Recommended remediation

• Patch to a fixed version listed in the vendor advisory (see references below).
• Mitigate with WAF rules, network egress filters, or feature flags where the patch is not yet available.
• Hunt historical logs for exploitation indicators — see Detection signatures below.

05Technical details

For the full vendor write-up, exploit chains, and reference implementations, see the references list in section 09.

06Detection signatures

Open the Sigma generator with a pre-filled prompt for this CVE to draft a starting detection in your stack of choice:

Open in Sigma generator →

07Related CVEs

No directly-cited follow-up CVEs in the KB record for this advisory. The references list in section 09 carries the vendor cross-references.

08Timeline

• Published: 2026-01-05T00:00:00.000Z
• Last modified: 2026-03-19T03:55:17.926Z
• Added to CISA KEV: 2026-03-18
• BOD 22-01 due: 2026-04-01

09References

• wiki.zimbra.com — https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories
• wiki.zimbra.com — https://wiki.zimbra.com/wiki/Security_Center
• wiki.zimbra.com — https://wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_Policy
• wiki.zimbra.com — https://wiki.zimbra.com/wiki/Zimbra_Releases/10.1.13#Security_Fixes
• wiki.zimbra.com — https://wiki.zimbra.com/wiki/Zimbra_Releases/10.0.18#Security_Fixes