How severe is CVE-2025-66376?

CVE-2025-66376 has a CVSS 3.1 base score of 7.2 (HIGH). EPSS estimates a 19.6% probability of exploitation in the next 30 days. It is listed in the CISA Known Exploited Vulnerabilities catalog.

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

CISA Known Exploited Vulnerabilities

Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability is confirmed as actively exploited and is listed in the CISA KEV catalog.

Added to KEV

2026-03-18

Remediation due

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.

CISA notes
https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories ; https://nvd.nist.gov/vuln/detail/CVE-2025-66376

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.

Severity

CVSS 3.1 · CNA7.2 HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

SSVC · CISA-ADP

Exploitationactive

Automatableno

Technical impactpartial

Affected Products

Vendor	Product	Affected versions
Zimbra	Collaboration	10.0 to <10.0.18 10.1 to <10.1.13

References

5 Links

Other references

Record Details

Published

2026-01-05

Last updated

2026-03-19

Assigner (CNA)

mitre

Credited to

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Zimbra Collaboration Suite (ZCS) OS Command Injection Vulnerability

Related Tool Categories

Tool categories that test for or protect against this vulnerability class.

CWE-79 · Cross-site Scripting

Application Security Tools Penetration Testing Tools

CVE record data © The MITRE Corporation, used under the CVE Terms of Use. Exploited-in-the-wild data from the CISA Known Exploited Vulnerabilities catalog (CC0). Exploit prediction scores by FIRST EPSS (first.org/epss). This site is not endorsed or certified by MITRE, NVD, CISA, or FIRST.