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Findings

CVE-2025-66376

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

Published Jan 5, 2026 · updated Oct 7, 2026 · Synacor Zimbra Collaboration Suite (ZCS)

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.

Timeline

Detection, updates, and resolution for this record.

  1. Jan 5, 2026 · Catalog

    Detected

    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.

  2. Mar 18, 2026 · CISA KEV

    Known exploited

    Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

  3. Apr 1, 2026 · CISA KEV

    Remediation due

    Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

  4. Oct 7, 2026 · Catalog

    Updated

    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.

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.

CISA due date Apr 1, 2026

Affected

  • cpe synacor/zimbra collaboration suiteversion unspecified
  • vendor Synacor Zimbra Collaboration Suite (ZCS)version unspecified

Sources

  • CISA KEVCVE-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.gov
  • CVE.orgCVE-2025-66376

    Official CVE identifier

    cve.org
  • FIRST EPSSCVE-2025-66376

    30-day exploitation probability 20.2%

    first.org
  • NIST NVDCVE-2025-66376

    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.

    nvd.nist.gov

References