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CVE-2025-66376

6.1, medium Known exploited

Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability

NVD (opens in a new tab) CVE.org (opens in a new tab) CISA KEV catalogue (opens in a new tab)
CVSS
6.1, medium medium CVSS v3.1
EPSS, next 30 days
20% higher than 97.4% of scored CVEs
Exploitation
Confirmed in the wild
Published
5 Jan 20269 months ago

Known exploited

CISA added this to the Known Exploited Vulnerabilities Catalog on 18 Mar 2026. US federal agencies had to remediate by 1 Apr 2026, a deadline that has passed.

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.

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.

Details

Vendor
Synacor
Product
Collaboration
Weakness
CWE-79 (opens in a new tab)
Status
Analyzed
Updated
7 Oct 2026
Assigner
cve@mitre.org

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

References 6

Advisories 4

Other references 2

The kind is read from NVD's tags where present and from the shape of the URL otherwise, so treat it as a hint.

Other Synacor CVEs

Every Synacor CVE we hold

Same vendor in what we hold, not a known link between the flaws.

  • CVE-2026-73570 Zimbra Collaboration Suite (ZCS) OS Command Injection Vulnerability KEV EPSS 72% 8.9, high
  • CVE-2025-68645 Synacor Zimbra Collaboration Suite (ZCS) PHP Remote File Inclusion Vulnerability KEV EPSS 49% 8.8, high
  • CVE-2025-48700 Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability KEV EPSS 2% 6.1, medium
  • CVE-2025-27915 Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability KEV EPSS 4% 5.4, medium
  • CVE-2024-45519 Synacor Zimbra Collaboration Suite (ZCS) Command Execution Vulnerability KEV EPSS 100% 9.8, critical
  • CVE-2024-27443 Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Vulnerability KEV EPSS 25% 6.1, medium
  • CVE-2023-37580 Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Vulnerability KEV EPSS 49% 6.1, medium
  • CVE-2023-34192 Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Vulnerability KEV EPSS 77% 9, critical

Beyond this page

Is CVE-2025-66376 in your network?

This page tells you how dangerous CVE-2025-66376 is out there. Whether it sits on one of your devices, only a scan of your own network can tell. That is what our systems do, every day, for the companies we protect.

CVE-2025-66376 is in the CISA catalogue of exploited vulnerabilities: attackers are already exploiting it.

  • EXYS7600 OCELOT

    Find it before attackers do

    Ocelot scans your network on a schedule and reports every vulnerability it finds by its CVE identifier, with its CVSS score and how many of your hosts are affected. A finding like CVE-2025-66376 comes with the devices to patch.

    Learn more about EXYS7600 OCELOT

  • EXYS7950 KONDRA

    Detect it if it's exploited

    Until the patch is applied, Kondra monitors your network, events and endpoints in real time. It keeps an inventory of the software installed on every endpoint, and its anomaly detection rules flag what an attacker does once inside.

    Learn more about EXYS7950 KONDRA
Check your exposure

Assembled from the NVD 2.0 API, the CISA Known Exploited Vulnerabilities Catalog and FIRST EPSS. Scores are as of our last collection on 17 Oct 2026, not the live upstream state.