CVE-2025-66376: Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability

What to do

1. Check whether your estate includes Synacor Zimbra Collaboration Suite (ZCS), including internet-facing devices.
2. Apply the vendor’s patch or mitigations (references below). Failing that, isolate the product or stop using it, as CISA requires of US federal agencies.
3. An exploited vulnerability has often been used before it was fixed: look for signs of compromise, especially on internet-facing devices.

References

• Vendor advisory or reference cited by CISA (wiki.zimbra.com)
• NVD entry (NIST)
• CISA KEV catalog