CVE-2025-66376: Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability
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.
| Vendor | Synacor |
|---|---|
| Product | Zimbra Collaboration Suite (ZCS) |
| Date added to KEV | 2026-03-18 |
| Due date (BOD 22-01) | 2026-04-01 |
| Known ransomware campaign use | Unknown |
| CWE | CWE-79 |
| Required action | Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. |
What to do
- Identify every instance of Synacor Zimbra Collaboration Suite (ZCS) in your environment (asset inventory, EDR, vulnerability scanner).
- Apply CISA's required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
- Federal civilian agencies must act by 2026-04-01 under BOD 22-01; other organizations can use the same date as a target.
- Check the vendor advisory and NVD entry linked below for affected versions, then record the fix in your change log.
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Sources
- NVD entry for CVE-2025-66376 (severity scores, if any, live there)
- CISA KEV catalog entry
- https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories
- https://nvd.nist.gov/vuln/detail/CVE-2025-66376