Zimbra Collaboration Suite Classic Web Client XSS (CVE-2025-27915)

Historical catalog analysis: CISA added this entry on October 07, 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-2025-27915 is a cross-site scripting (XSS) vulnerability identified in the Synacor Zimbra Collaboration Suite (ZCS). The flaw exists within the Classic Web Client due to insufficient sanitization of HTML content found in ICS files. An attacker can embed malicious JavaScript within an ICS entry that executes via an ontoggle event when a user views the associated email message.

If successfully executed, this arbitrary JavaScript runs within the victim’s active session. This could allow an unauthorized actor to perform actions on behalf of the user, specifically the creation of email filters to redirect messages to an external address or the exfiltration of account data.

Exposure and applicability

This vulnerability specifically affects organizations utilizing the ZCS Classic Web Client. The attack vector is a malicious ICS file delivered via email; therefore, any user accessing their mail through the affected client who views such a file is at risk.

Because this vulnerability has been added to CISA’s Known Exploited Vulnerabilities (KEV) catalog as of October 7, 2025, it is confirmed to be an active threat. Federal agencies are subject to a remediation deadline of October 28, 2025, though the risk applies to all ZCS Classic Web Client deployments regardless of sector.

Remediation priorities

Our analysis suggests that vulnerability management teams should prioritize this based on its presence in the KEV catalog and the potential for silent data exfiltration via email redirection.

1. Immediate Patching: The primary corrective action is to apply mitigations according to vendor instructions provided by Synacor Zimbra.
2. Client Review: Organizations should identify users still relying on the Classic Web Client, as this is the specific component where the sanitization failure occurs.
3. Session Monitoring: While not a preventative fix for the XSS itself, reviewing recent changes to email forwarding rules and filters across the environment may help identify if the vulnerability has already been leveraged.

How to validate remediation

Verification of this fix requires more than a version check. To ensure exposure is reduced, defenders should:

• Confirm Vendor Mitigation Application: Verify that the specific vendor-supplied updates addressing CVE-2025-27915 have been successfully deployed across all ZCS nodes.
• Functional Validation: In a controlled, non-production environment, test whether the Classic Web Client still processes ontoggle events within ICS files. A successful mitigation should result in the sanitization or blocking of such JavaScript execution.

Limits and open questions

It remains unknown if this vulnerability has been utilized in ransomware campaigns. Additionally, while vendor mitigations are available, the effectiveness of these controls depends on complete deployment across all affected infrastructure. Residual risk persists if users can bypass updated clients or if legacy configurations prevent the application of the latest security updates.

Source and editorial note

CVE-2025-27915: Synacor Zimbra Collaboration Suite (ZCS) Cross-site Scripting Vulnerability · Source date: October 07, 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: October 10, 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: September 30, 2026 at 01:24 UTC.

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