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CISA KEV · ACTIVELY EXPLOITED
HIGH severity

CVE-2025-66376 — zm-build

HIGH

CVE-2025-66376 is a high-severity (CVSS 7.2) Cross-site Scripting (XSS) vulnerability. It is in CISA's Known Exploited Vulnerabilities catalog (added 2026-03-18) — treat it as actively exploited and patch now. A fix is available — see the affected versions and patch details below.

Published
Updated
Affected
1 product
Patched
See advisory
Exploits
None indexed
Exploitation data as of Oct 23, 2026 · OSV.dev, NVD, CISA KEV, FIRST.org (EPSS)

Exploitation Status

Actively exploited in the wild

  • Confirmed by CISA's Known Exploited Vulnerabilities catalog on 2026-03-18. Federal agencies were required to remediate by 2026-04-01.

Exploitation and automatability from CISA (KEV catalog and SSVC triage) for CVE-2025-66376.

EPSS Exploitation Probability

via FIRST.org ↗
20.2%probability of exploitation in next 30 days
Moderate Risk+0.67%
Lower risk than most CVEs97th percentile — riskier than 97% of all scored CVEsHighest risk
7.38%13.2%19.1%24.9%10.7%21.6%19.6%20.2%May 26Oct 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2025-66376 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 395,249 CVEs with a current EPSS score, this one falls in the 10–50% band (highlighted). Counts from FIRST.org, log-scaled.

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.

Affected Products

1 product · 2 configurations
Application
zimbra collaboration suitesynacor
≥ 10.1.0 && < 10.1.13
range

Detection & mitigation playbook

Vulnerability

  1. Detect

    Identify every host running the affected component and compare the installed build against the fixed version below — for source-built or distro-packaged software the version string, not a lockfile, is the source of truth (`dpkg -l`, `rpm -q`, or the binary's own `--version`).

  2. Fix

    Upgrade the affected component to the fixed release for CVE-2025-66376, or apply your distribution's backported patch — distro builds are often patched at an older version number, so check your vendor's advisory rather than the upstream version alone.

  3. Workarounds

    Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

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.
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CVE-2025-66376: zm-build XSS (High 7.2, KEV)