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.
- Vendor
- Synacor
- Product
- Zimbra Collaboration Suite (ZCS)
- CVSS
- 7.2
- EPSS (exploit probability)
- 20.2%
- Status
- kev
- CISA patch-by (BOD 22-01)
- Added to KEV
Is CVE-2025-66376 being exploited?
Yes. CISA added CVE-2025-66376 to its Known Exploited Vulnerabilities catalog on . CISA only lists a CVE when it has reliable evidence of exploitation in the wild.
FIRST's EPSS model puts the probability of exploitation activity in the next 30 days at 20.2% (scored October 14, 2026).
- CISA KEV[ LISTED ]
- ENISA EUVD (exploited)[ — ]
- CIRCL KEV[ — ]
Catalog snapshot 2026-10-21. Full cross-catalog view: /kev-comparison
What to do
CISA required actionApply 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 civilian agencies must remediate by (14 days after listing) under BOD 22-01. Everyone else: that deadline is a sensible upper bound, not a target. Vendor guidance and the NVD record are linked under Source above.
Other Synacor Zimbra Collaboration Suite (ZCS) CVEs we track
| CVE / Vulnerability | Severity | CVSS | EPSS | Added to KEV |
|---|---|---|---|---|
| CVE-2026-73570 Zimbra ZCS SNMP Command Injection — Unauthenticated RCEwrite-up | [ HIGH ] | 8.9 | 71.7% | |
| CVE-2025-48700 Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability | [ MEDIUM ] | 6.1 | 1.7% | |
| CVE-2020-7796 Synacor Zimbra Collaboration Suite (ZCS) Server-Side Request Forgery Vulnerability | [ CRITICAL ] | 9.8 | 84.4% | |
| CVE-2025-68645 Synacor Zimbra Collaboration Suite (ZCS) PHP Remote File Inclusion Vulnerability | [ HIGH ] | 8.8 | 48.9% | |
| CVE-2025-27915 Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability | [ MEDIUM ] | 5.4 | 4.1% | |
| CVE-2019-9621 Synacor Zimbra Collaboration Suite (ZCS) Server-Side Request Forgery (SSRF) Vulnerability | [ HIGH ] | 7.5 | 81.0% | |
| CVE-2024-27443 Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Vulnerability | [ MEDIUM ] | 6.1 | 24.6% | |
| CVE-2023-34192 Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Vulnerability | [ CRITICAL ] | 9.0 | 77.3% |