CISA Adds Actively Exploited Zimbra XSS Flaw to KEV Catalog
CISA added CVE-2025-66376 to its Known Exploited Vulnerabilities (KEV) catalog after confirming active exploitation of a stored cross-site scripting (XSS) flaw in Synacor Zimbra Collaboration Suite (ZCS). The vulnerability affects the Classic UI and allows remote unauthenticated attackers to abuse CSS @import directives embedded in email HTML, creating a path to execute malicious JavaScript in a victim's browser session. The KEV entry describes the issue as a CWE-79 XSS vulnerability and directs organizations to apply vendor mitigations, follow BOD 22-01 guidance for cloud services, or discontinue use if mitigations are unavailable.
CISA ordered Federal Civilian Executive Branch (FCEB) agencies to remediate the flaw by April 1, while also urging private-sector organizations to patch quickly because the bug is being exploited in the wild. Reporting on the KEV addition notes that the flaw was patched by Zimbra earlier and could enable session hijacking and theft of sensitive data within affected Zimbra environments through malicious HTML email content. The same KEV update also included other unrelated vulnerabilities, but the Zimbra entry is the relevant event tied to the active exploitation warning and federal patching directive.
Timeline
Mar 18, 2026
CISA orders federal agencies to patch exploited Zimbra flaw
CISA ordered U.S. federal civilian agencies to remediate CVE-2025-66376 under Binding Operational Directive 22-01 after confirming active exploitation in the wild. Federal Civilian Executive Branch agencies were given until 2026-04-01 to secure affected Zimbra servers, while private-sector organizations were urged to patch immediately.
Mar 18, 2026
CISA adds CVE-2025-66376 to the KEV catalog
CISA added the actively exploited Zimbra flaw CVE-2025-66376 to its Known Exploited Vulnerabilities catalog as part of the March 18, 2026 KEV update. The catalog entry directed organizations to apply vendor mitigations or discontinue use if no fix is available.
Nov 1, 2025
Synacor patches Zimbra stored XSS flaw CVE-2025-66376
Synacor released a fix for CVE-2025-66376 in Zimbra Collaboration Suite in early November 2025. The high-severity stored XSS bug affects the Classic UI and can be triggered via malicious HTML emails using CSS @import directives.
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