How severe is CVE-2025-66376?

CVE-2025-66376 has a CVSS 3.x base score of 7.2, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity low, and availability none.

Is CVE-2025-66376 being actively exploited?

Yes. CVE-2025-66376 is on CISA's Known Exploited Vulnerabilities (KEV) catalog, added on 2026-03-18, which means active exploitation has been confirmed. It should be prioritised for remediation.

What products are affected by CVE-2025-66376?

CVE-2025-66376 affects Synacor Zimbra Collaboration Suite. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2025-66376?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Because this CVE is known to be actively exploited, treat remediation as urgent — CISA KEV typically sets a short remediation deadline.

Does CVE-2025-66376 have an EU (EUVD) identifier?

Yes. CVE-2025-66376 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-0850. It is also flagged as exploited in the EUVD (since 2026-03-18).

When was CVE-2025-66376 published?

CVE-2025-66376 was published on 2026-01-05 and last updated on 2026-06-17.

CVE-2025-66376

CVE-2025-66376 is a high-severity vulnerability in Synacor Zimbra Collaboration Suite with a CVSS 3.x base score of 7.2. It is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, confirming it has been exploited in the wild (added 2026-03-18). The underlying weakness is classified as CWE-79.

Key facts

Severity: High (CVSS 3.x base score 7.2)
EPSS exploit prediction: 12% (96th percentile)
Actively exploited: Yes — listed in CISA KEV (added 2026-03-18)
EU (EUVD) id: EUVD-2026-0850
EU exploitation: Flagged exploited in the ENISA EU Vulnerability Database (since 2026-03-18)
Weakness: CWE-79
Affected product: Synacor Zimbra Collaboration Suite
Published: 2026-01-05
Last modified: 2026-06-17

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.

Frequently asked questions

What is 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.
How severe is CVE-2025-66376?: CVE-2025-66376 has a CVSS 3.x base score of 7.2, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity low, and availability none.
Is CVE-2025-66376 being actively exploited?: Yes. CVE-2025-66376 is on CISA's Known Exploited Vulnerabilities (KEV) catalog, added on 2026-03-18, which means active exploitation has been confirmed. It should be prioritised for remediation.
What products are affected by CVE-2025-66376?: CVE-2025-66376 affects Synacor Zimbra Collaboration Suite. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2025-66376?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Because this CVE is known to be actively exploited, treat remediation as urgent — CISA KEV typically sets a short remediation deadline.
Does CVE-2025-66376 have an EU (EUVD) identifier?: Yes. CVE-2025-66376 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-0850. It is also flagged as exploited in the EUVD (since 2026-03-18).
When was CVE-2025-66376 published?: CVE-2025-66376 was published on 2026-01-05 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:synacor:zimbra_collaboration_suite:*:*:*:*:*:*:*:*

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