What is CVE-2025-66376?

CVE-2025-66376 is a high-severity vulnerability in Synacor Zimbra Collaboration Suite, classified under Cross-site Scripting. CVSS score: 7.2/10. Published 2026-01-05.

How severe is CVE-2025-66376?

High severity. CVSS v3 base score is 7.2 out of 10.

Is CVE-2025-66376 known to be exploited?

Yes. CVE-2025-66376 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-03-18), indicating it is being actively exploited.

XSS in Synacor Zimbra Collaboration Suite

CVE-2025-66376

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.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2026-01-05 · last modified 2026-10-07.

CVSS v3 metric

CVSS v3 base score 7.2 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.202 (97.4th percentile), scored 2026-10-18.

Moderate probability of exploitation in the next 30 days — watch for active exploitation reports. 97th percentile — 97.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.196 · newest: 0.202 · change: +0.007

Affected products

Synacor Zimbra Collaboration Suite
Zimbra Collaboration — versions 10.0, 10.1

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

CISA KEV (Known Exploited Vulnerabilities)

This CVE is on the CISA KEV catalog, added on 2026-03-18. CISA KEV inclusion means CISA has confirmed in-the-wild exploitation; US federal agencies are required to remediate within a published due date.

BOD 22-01 due date: 2026-04-01.

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.

References

wiki.zimbra.com/wiki/Zimbra_Security_Advisories (Vendor Advisory)
wiki.zimbra.com/wiki/Security_Center (Release Notes, Vendor Advisory)
wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_Policy (Product)
wiki.zimbra.com/wiki/Zimbra_Releases/10.1.13 (Release Notes)
wiki.zimbra.com/wiki/Zimbra_Releases/10.0.18 (Release Notes)
134c704f-9b21-4f2e-91b3-4a467353bcc0 (US Government Resource)

Frequently asked questions

What is CVE-2025-66376?: CVE-2025-66376 is a high-severity vulnerability in Synacor Zimbra Collaboration Suite, classified under Cross-site Scripting. CVSS score: 7.2/10. Published 2026-01-05.
How severe is CVE-2025-66376?: High severity. CVSS v3 base score is 7.2 out of 10.
Is CVE-2025-66376 known to be exploited?: Yes. CVE-2025-66376 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-03-18), indicating it is being actively exploited.