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Exploited vulnerability · CISA KEV

Is CVE-2025-66376 exploited? Yes: in CISA KEV since 18 Mar 2026

Gociux · Synacor Zimbra Collaboration Suite (ZCS) · data: CISA KEV, CVE record, FIRST EPSS

Yes. CVE-2025-66376, Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability, is being exploited in the wild. CISA added it to its Known Exploited Vulnerabilities catalog on 18 Mar 2026, 72 days after it was published. It affects Synacor Zimbra Collaboration Suite (ZCS). If you run it, CISA's required action is: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

exploitedYesCISA KEV · 18 Mar 2026
ransomwareNot knownno confirmed use
EPSStop 5%very likely to be exploited
severity7.2CVSS 3.1 · high
US federal due1 Apr 2026BOD 22-01 deadline

What it is

Synacor Zimbra Collaboration Suite (ZCS) contains a cross-site scripting vulnerability in the Classic UI where attackers could abuse Cascading Style Sheets (CSS) @import directives in email HTML.

VendorSynacor (19 exploited CVEs in KEV)
ProductZimbra Collaboration Suite (ZCS)
VulnerabilitySynacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability
WeaknessCWE-79
Published5 Jan 2026
Added to KEV18 Mar 2026
Reported by (CNA)mitre

What to do

  1. Check whether you run it. Look for Zimbra Collaboration Suite (ZCS) in your asset inventory, including appliances and cloud images. Stack check lists every exploited Synacor CVE at once.
  2. Apply the fix. Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
  3. If you can't patch today, take the affected service off the internet or restrict who can reach it, and check its logs for signs of compromise since 5 Jan 2026.
  4. Get told next time. Email alerts when CISA adds a CVE for Synacor.

Other exploited CVEs in Zimbra Collaboration Suite (ZCS)

References

Questions

Is CVE-2025-66376 being exploited?

Yes. CISA added CVE-2025-66376 to the Known Exploited Vulnerabilities catalog on 18 Mar 2026, which means there is reliable evidence of exploitation in the wild.

Is CVE-2025-66376 used in ransomware attacks?

CISA does not list known ransomware use for it. That can change, and exploitation alone is reason to fix it first.

How do I fix CVE-2025-66376?

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. Check Synacor's advisory for the fixed versions, and if you can't patch quickly, restrict access to the affected Zimbra Collaboration Suite (ZCS) and watch it for signs of compromise.

Sources: CISA KEV (exploitation, ransomware use, required action, due date), the CVE record (publication date, CVSS, CNA), NVD and FIRST EPSS (live score). More: all KEV additions in March 2026 · Synacor scorecard · act-now radar.

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