CVE-2021-35209: Zimbra Collaboration

Critical severity, CVSS 9.8. EPSS: 3% chance of exploitation in the next 30 days.

An issue was discovered in ProxyServlet.java in the /proxy servlet in Zimbra Collaboration Suite 8.8 before 8.8.15 Patch 23 and 9.x before 9.0.0 Patch 16. The value of the X-Host header overwrites the value of the Host header in proxied requests. The value of X-Host header is not checked against the whitelist of hosts Zimbra is allowed to proxy to (the zimbraProxyAllowedDomains setting).

Affected products

• Zimbra Collaboration: from 8.8, before 8.8.15 (fixed in 8.8.15); version 8.8.15 only; version 9.0.0 only

Published 2021-07-02. Last modified 2026-06-17.