How severe is CVE-2025-66376?

CVE-2025-66376 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2025-66376?

Affected package information for CVE-2025-66376 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2025-66376?

No fixed version has been published for CVE-2025-66376 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2025-66376 actively exploited in the wild?

No public exploit code has been indexed for CVE-2025-66376 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2025-66376?

CVE-2025-66376 has an EPSS (Exploit Prediction Scoring System) score of 10.9%, placing it in the 94th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score warrants monitoring and prompt remediation.

Is CVE-2025-66376 in the CISA Known Exploited Vulnerabilities catalog?

Yes. CVE-2025-66376 (Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability) was added to CISA's Known Exploited Vulnerabilities (KEV) catalog on 2026-03-18. CISA required all US federal civilian agencies to remediate this vulnerability by 2026-04-01. The required action was: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. Being listed in the KEV catalog is the strongest possible confirmation of active exploitation — treat this as a critical priority.

When was CVE-2025-66376 published, and has it been updated?

CVE-2025-66376 was published on January 5, 2026 and was last updated on April 10, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CISA KEV·Added 2026-03-18 — agencies required to remediate by 2026-04-01

CVE-2025-66376

MEDIUM

Published

Jan 5, 2026

Updated

Apr 10, 2026

Affected

0 pkgs

Patched

None yet

Exploits

None indexed

EPSS Exploitation Probability

via FIRST.org ↗

10.9%probability of exploitation in next 30 days

Moderate Risk94th percentile0.00%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

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

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.

O3 Security · Impact-Aware SCA

Is CVE-2025-66376 in your stack?

O3 detects CVE-2025-66376 across dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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