Zimbra Collaboration Suite (ZCS) XSS via CSS @import

Historical catalog analysis: CISA added this entry on March 18, 2026. The entry reflects catalog information retrieved on August 31, 2026, not a snapshot archived on the inclusion date. Check current vendor guidance before acting.

What the vulnerability is

CVE-2025-66376 is a cross-site scripting (XSS) vulnerability identified in the Synacor Zimbra Collaboration Suite (ZCS). The flaw exists within the product’s Classic UI, where the system fails to properly handle Cascading Style Sheets (CSS) @import directives embedded in email HTML. This allows an attacker to execute arbitrary scripts in the context of the user’s session by sending a specially crafted email.

Exposure and applicability

This vulnerability specifically affects ZCS deployments utilizing the Classic UI. Organizations using alternative or updated UI versions may not be exposed, though the source does not explicitly list non-affected versions. The primary attack vector is an inbound email containing malicious HTML/CSS; therefore, any user accessing their mail via the Classic UI is a potential target.

Remediation priorities

Based on our analysis, vulnerability management teams should prioritize the following actions to reduce exposure:

1. Asset Identification: Determine which ZCS instances are active and specifically identify if the Classic UI is enabled or in use by the user base.
2. Vendor Mitigation Application: Apply the specific mitigations provided in the Synacor vendor instructions. This is the primary method for addressing the underlying flaw.
3. Cloud Service Review: For organizations utilizing ZCS via cloud providers, review service configurations against BOD 22-01 guidance to ensure provider-side mitigations are active.
4. Decommissioning Evaluation: In scenarios where vendor mitigations cannot be applied or are unavailable for a specific legacy version, evaluate the feasibility of discontinuing the use of the affected product component.

How to validate remediation

Verification must go beyond a simple version check, as a deployed patch does not always guarantee that the configuration is secure. To verify that exposure has been reduced, defenders should:

• Confirm Mitigation Deployment: Audit system logs and update records to ensure vendor-supplied mitigations were applied successfully across all affected nodes.
• Functional Validation: In a controlled, authorized test environment, attempt to trigger the vulnerability using a benign CSS @import directive in an email to confirm that the Classic UI no longer processes the directive as intended.
• Configuration Audit: Verify that any compensating controls or configuration changes recommended by the vendor are active and enforced across the environment.

Limits and open questions

It remains unknown whether this vulnerability is being actively exploited by ransomware campaigns. Additionally, while CISA has established a remediation deadline for federal agencies (2026-04-01), this date does not automatically apply to private sector organizations. There is residual risk if users are permitted to bypass updated interfaces to access the Classic UI, or if vendor mitigations are applied inconsistently across a distributed environment.

Source and editorial note

CVE-2025-66376: Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting Vulnerability · Source date: March 18, 2026 · Retrieved August 31, 2026.

Material facts are tied to the cited primary source. Recommendations are independent defensive analysis unless attributed to the source. Confirm consequential decisions against current authoritative guidance.

Archive date: March 21, 2026. The displayed post date is assigned three days after the source date to organize this retrospective archive; it does not mean this site published the analysis then. First published by this site: September 18, 2026 at 00:10 UTC.

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