Zimbra Collaboration Suite CVE-2024-27443 XSS Exposure

Historical catalog analysis: CISA added this entry on May 19, 2025. 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-2024-27443 is a Cross-Site Scripting (XSS) vulnerability identified in the Synacor Zimbra Collaboration Suite (ZCS). The flaw exists within the CalendarInvite feature of the webmail classic user interface. An attacker can trigger this vulnerability by sending an email message that contains a specifically crafted calendar header, which allows for the execution of arbitrary JavaScript code in the context of the user’s session.

Exposure and applicability

This vulnerability applies to organizations utilizing the Zimbra Collaboration Suite with the webmail classic UI enabled. The attack vector is remote and asynchronous; it requires no prior authentication but relies on a user interacting with a malicious email message.

Exposure is confirmed for versions of ZCS that have not been updated to the specific security patch levels identified by the vendor. Because this vulnerability has been added to CISA’s Known Exploited Vulnerabilities (KEV) catalog, it should be treated as a high-priority exposure regardless of whether internal scanning has detected active attempts.

Remediation priorities

Our analysis indicates that remediation must focus on updating the ZCS installation to a version containing the security fix. Based on vendor documentation, the following versions include the necessary mitigations:

• Zimbra 8.8.15: Update to Patch 46 (P46).
• Zimbra 9.0.0: Update to Patch 39 (P39).
• Zimbra 10.0.7: Apply the corresponding security updates for this version.

Infrastructure owners should prioritize these updates based on the visibility of their webmail classic UI to external or untrusted email sources. If patching is not immediately feasible, defenders should evaluate if the affected component can be isolated or if compensating controls at the mail gateway can identify and strip anomalous calendar headers, though such measures may not cover all exploit variations.

How to validate remediation

To verify that exposure has been reduced, vulnerability management teams must move beyond simple version checks. While confirming the installation of P46 (for 8.8.15) or P39 (for 9.0.0) is a primary step, validation should include:

1. Patch Level Verification: Confirming via the ZCS administration console or command line that the specific patch level—not just the base version—is active.
2. Configuration Audit: Verifying if the webmail classic UI is still in use and ensuring the update has been applied across all nodes in a clustered environment.

Verification is complete when the system state matches the vendor’s patched release specifications. A successful patch application indicates that the specific input handling flaw in the CalendarInvite feature has been addressed.

Limits and open questions

Applying these patches addresses the known XSS vector in the CalendarInvite feature but does not eliminate all risks associated with XSS or other vulnerabilities within the ZCS ecosystem. Residual risk remains if users are running outdated browser versions that lack modern XSS protections (such as Content Security Policy enforcement).

It remains unknown whether this vulnerability has been leveraged by specific ransomware campaigns, and the full extent of potential downstream impacts following JavaScript execution—such as session hijacking or data exfiltration—depends on the specific environment’s security posture.

Source and editorial note

CVE-2024-27443: Synacor Zimbra Collaboration Suite (ZCS) Cross-Site Scripting (XSS) Vulnerability · Source date: May 19, 2025 · 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: May 22, 2025. 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: October 14, 2026 at 00:32 UTC.

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