Vulnerability in Zimbra Collaboration Suite (ZCS)

[ Why it matters ]Priority signals

• Critical severity: CVSS 9.3

[ Description ]Official description

An unauthenticated calendar sender can place active markup in a COUNTER message's RFC From address. Selecting the message in Zimbra Classic triggers stored XSS, allowing the attacker to access mailbox data and act as the victim.

[ Fixes ]Fixes and patches

No fix or interim patch is linked from our sources yet. Check the vendor references below and limit exposure of affected systems until a fix is available.

[ Advisories ]Government CERT advisories

• BSI CERT-Bund27 Sep 2026WID-SEC-2026-3593

  Synacor Zimbra: Multiple vulnerabilities Translated from German

  CVE-2026-93647CVE-2026-66912CVE-2026-66911+3 more CVEs

[ References ]References

• wiki.zimbra.com/wiki/Zimbra_Security_Advisories