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.
Code
The project this record applies to. Exploit listings stay in References.
No public source repository is identified for this product.
Timeline
Detection, updates, and resolution for this record.
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.
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.
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.
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 KEVConfirmed exploitationCVE-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.orgVulnerability recordCVE-2025-66376
Official CVE identifier
cve.org - FIRST EPSSExploitation probabilityCVE-2025-66376
30-day exploitation probability 20.2%
first.org - NIST NVDVulnerability recordCVE-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