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.

Added to CISA’s Known Exploited Vulnerabilities catalog on 2026-03-18. Required action per CISA: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. Due date: 2026-04-01.

This is an auto-synced KEV catalog record pending a full 0dayNews write-up — see CISA’s KEV catalog and the NVD record linked above for primary sources.