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[ CVE-2025-66376 · Published 5 Jan 2026 ]

Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability

Exploited · CISA KEV CVSS 6.1 EPSS 20%No fix listed yet

[ Why it matters ]Priority signals

  • Exploited in the wild: on CISA's Known Exploited Vulnerabilities list since 2026-03-18
  • EPSS: 20% chance of exploitation in the next 30 days (higher than 97% of CVEs)

[ Description ]Official description

Zimbra Collaboration (ZCS) 10 before 10.0.18 and 10.1 before 10.1.13 allows Classic UI stored XSS via Cascading Style Sheets (CSS) @import directives in an HTML e-mail message.

CISA: 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.

[ Fixes ]Fixes and patches

CISA required action (US federal deadline 1 Apr 2026): Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

No fix or interim patch is linked from our sources yet. Check the vendor references below and limit exposure of affected systems until a fix is available.

[ References ]References