Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability (CVE-2025-66376)
Severity: medium · Patch status: unknown · Since 2026-03-18 · CVE-2025-66376
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. CISA added CVE-2025-66376 to its Known Exploited Vulnerabilities catalog on Mar 18, 2026, which means attackers are exploiting it in the wild.
What to do
- Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
- US federal agencies must remediate by Apr 1, 2026 (BOD 22-01); treat that as the outer limit.
Sources
All live incidents on Threat Watch