CVE-2025-66376

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

Description

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.

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.

CVSS 3.1 vectorCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:N

PublishedJan 5, 2026

Last modifiedOct 7, 2026

Added to KEVMar 18, 2026

Remediation dueApr 1, 2026

References

wiki.zimbra.com/wiki/Security_CenterRelease NotesVendor Advisory
wiki.zimbra.com/wiki/Zimbra_Releases/10.0.18Release Notes
wiki.zimbra.com/wiki/Zimbra_Releases/10.1.13Release Notes
wiki.zimbra.com/wiki/Zimbra_Responsible_Disclosure_PolicyProduct
wiki.zimbra.com/wiki/Zimbra_Security_AdvisoriesVendor Advisory
cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2025-66376US Government Resource

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Risk assessment · High

Actively exploited. Patch immediately.

Exploitation: Confirmed in the wild (CISA KEV)
Federal remediation deadline: Overdue by 210 days
CVSS: 7.2 HIGH
EPSS (30-day exploit probability): 20.2% · top 3% of all CVEs
Weakness: CWE-79
Attack vector: Network, remotely reachable
Preconditions: No privileges and no user interaction

Recommended actions

1.Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
2.Treat as emergency change: patch or isolate internet-facing instances first.
3.Review exposure: confirm which instances are reachable from the internet and restrict access until patched.
4.Hunt for compromise: review logs for the disclosure window, not just from the patch date.

Assessment is derived automatically from the live signals above; verify against your own asset inventory.