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CISA Warns of Zimbra Collaboration Suite Vulnerability Exploited in Attacks

## Zimbra Collaboration Suite Vulnerability Update

ABy Adam FosterMar 19, 20261 min read

Zimbra Collaboration Suite Vulnerability Update

The Cybersecurity and Infrastructure Security Agency (CISA) has identified a high-severity vulnerability in the Zimbra Collaboration Suite (ZCS), now included in their Known Exploited Vulnerabilities catalog.

Identified as CVE-2025-66376, this vulnerability is subject to active exploitation. Organizations using Zimbra are advised to prioritize remediation to prevent unauthorized access and data compromise.

The security flaw is a stored cross-site scripting (XSS) issue within the Classic User Interface of ZCS. Threat actors can exploit this by sending emails containing specially formatted code. This attack method utilizes Cascading Style Sheets (CSS) @import directives within the HTML email body.

Upon opening the malicious email in the Classic UI, scripts execute automatically within the user's active session. This allows potential unauthorized actions, such as session cookie harvesting and unauthorized command execution.

Identified as CVE-2025-66376, this vulnerability is subject to active exploitation.
Adam Foster · Thehackingpost

Zimbra has addressed this vulnerability in recent updates: versions 10.1.13 and 10.0.18. These patches eliminate the XSS vulnerability. Additionally, Zimbra updated the AntiSamy security library to version 1.7.8 and removed deprecated code.

Version 10.1.13 also includes enhancements in user experience and performance, such as improved TLS handling, optimized memory management, and faster email thread loading.

User interface improvements include enhanced drag-and-drop file management, consistent copy-paste formatting from Microsoft Office, and better tag organization. Compatibility with Outlook 2024 and Legacy Exchange Web Services (EWS) is maintained.

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CISA Directive and End of Life Advisory

CISA mandates all Federal Civilian Executive Branch agencies apply the relevant Zimbra patches by April 1, 2026. Private entities are encouraged to adhere to this deadline as well. If patches cannot be applied, discontinuation of the vulnerable product is recommended.

Administrators should note that Zimbra version 10.0 reached its End of Life on December 31, 2025. Organizations using this version should plan an immediate migration to Zimbra 10.1 to ensure security compliance, as continued use of an EOL platform exposes systems to future vulnerabilities.

Based on reporting by Cyber Security News.

AI transparency. This article was produced with the assistance of artificial intelligence and published under human editorial oversight. AI systems can make mistakes. Read how we use AI (EU AI Act, Art. 50).
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