CVE-2021-35209

An issue was discovered in ProxyServlet.java in the /proxy servlet in Zimbra Collaboration Suite...

9.8 Critical

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).

Attack profile

How easy it is to exploit

Attack vector: Network
Complexity: Low
Privileges needed: None
User interaction: None

What an attacker can affect

Confidentiality: High
Integrity: High
Availability: High

Exploit likelihood

3.00%Up 2.0 points since Jul 3, 2021

What each source reports

NVD Score used 9.8

Record: CVE-2021-35209
CVSS 3.1: 9.8 CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CVSS 2.0: 7.5 HighAV:N/AC:L/Au:N/C:P/I:P/A:P
Weakness: CWE-918
Updated: 2026-11-02

CVE List No score

Record: CVE-2021-35209
Updated: 2026-11-02

EUVDEUVD-2021-21853 No score

Record: EUVD-2021-21853
Updated: 2026-11-03

GitHub AdvisoriesGHSA-wv5j-j74p-fv28 No score

An issue was discovered in ProxyServlet.java in the /proxy servlet in Zimbra Collaboration Suite...

Record: GHSA-wv5j-j74p-fv28
Weakness: CWE-601, CWE-918
Updated: 2026-11-02

OSV 9.8

Record: CVE-2021-35209
CVSS 3.1: 9.8 CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Updated: 2026-11-02

Affected products

zimbra collaboration

History

History
Recorded	Changed	Sources
Nov 3, 2026	aliases	EUVD
Nov 2, 2026	first indexed	CVE List, GitHub Advisories, NVD, OSV

Dates are when Every CVE recorded each change. Upstream dates may differ.

In short

CVE-2021-35209 is a critical vulnerability (CVSS 9.8) in Zimbra Collaboration, published July 2, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 3.00%, the 87th percentile. No source names a fixed version.