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PatchSiren cyber security CVE debrief

CVE-2025-66376 Synacor CVE debrief

CVE-2025-66376 is a cross-site scripting issue affecting Synacor Zimbra Collaboration Suite (ZCS) that CISA added to its Known Exploited Vulnerabilities catalog on 2026-03-18. Because it is flagged as known exploited, organizations running ZCS should treat it as a high-priority defensive item and follow vendor guidance and CISA remediation expectations.

Vendor
Synacor
Product
Zimbra Collaboration Suite (ZCS)
CVSS
HIGH 7.2
CISA KEV
Listed
Original CVE published
2026-03-18
Original CVE updated
2026-03-18
Advisory published
2026-03-18
Advisory updated
2026-03-18

Who should care

Administrators and security teams responsible for Synacor Zimbra Collaboration Suite (ZCS), especially internet-facing or externally accessible deployments, should prioritize this advisory. Email and collaboration platform owners, vulnerability management teams, and incident response staff should also review exposure and remediation status.

Technical summary

The supplied source corpus identifies CVE-2025-66376 as a cross-site scripting vulnerability in Synacor Zimbra Collaboration Suite (ZCS). CISA’s KEV entry indicates the issue is known to be exploited in the wild, but the corpus does not include further technical detail, exploit conditions, affected component specifics, or vendor remediation text.

Defensive priority

High. CISA’s KEV inclusion means this vulnerability should be prioritized ahead of routine issues, with attention to any available vendor mitigations or updates and exposure reduction for affected ZCS instances.

Recommended defensive actions

  • Review Synacor Zimbra Security Advisories for vendor instructions and remediation guidance.
  • Apply available mitigations or updates per vendor guidance as soon as possible.
  • Inventory all Zimbra Collaboration Suite (ZCS) deployments, including externally reachable instances.
  • If mitigations are unavailable, follow CISA guidance to discontinue use of the product.
  • Track remediation status against the CISA KEV due date of 2026-04-01.
  • Monitor affected environments for anomalous web activity and signs of user-session abuse consistent with client-side attack paths.

Evidence notes

This debrief is based on the supplied CISA KEV source item and official reference links only. The corpus confirms the product, vulnerability type, KEV status, and dates added to the catalog, but it does not include a CVSS score, vendor advisory text, or deeper technical root-cause details. Timing context uses the supplied CVE and KEV dates, with 2026-03-18 as the published/added date.

Sources and references

Verified primary and authoritative sources

  • CVE-2025-66376 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2025-66376

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2025-66376 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2025-66376

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

  • CISA Known Exploited Vulnerabilities catalog

    Publisher, destination, and source semantics verified

    URL: https://www.cisa.gov/known-exploited-vulnerabilities-catalog

    Cybersecurity and Infrastructure Security Agency - Official CISA catalog of vulnerabilities known to be exploited in the wild.

Supplemental references

  • Source item URL

    Unverified legacy reference

    URL: https://www.cisa.gov/sites/default/files/feeds/known_exploited_vulnerabilities.json

    cisa_kev

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.