CVE-2021-35209

Name: CVE-2021-35209
Creator: NVD / NIST
Published: 2021-07-02T19:15:08.46+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 9.8/10EPSS 2.97%

Last modified Jun 17, 2026

CVE-2021-35209 is a critical-severity vulnerability rated 9.8/10 on the CVSS scale. 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. EPSS estimates a 2.97% chance of exploitation in the next 30 days.

Description

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).

Metrics

CVSS 3.1

9.8/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS Probability

2.97%

85.5th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-918

Affected Software

Vendor	Product	Versions
Zimbra	Collaboration	>= 8.8, < 8.8.15
Zimbra	Collaboration	8.8.15
Zimbra	Collaboration	9.0.0

References

https://blog.sonarsource.com/zimbra-webmail-compromise-via-emailExploit, Third Party Advisory
https://wiki.zimbra.com/wiki/Security_CenterRelease Notes, Vendor Advisory
https://wiki.zimbra.com/wiki/Zimbra_Releases/8.8.15/P23Release Notes, Vendor Advisory
https://wiki.zimbra.com/wiki/Zimbra_Releases/9.0.0/P16Release Notes, Vendor Advisory
https://wiki.zimbra.com/wiki/Zimbra_Security_AdvisoriesVendor Advisory
https://blog.sonarsource.com/zimbra-webmail-compromise-via-emailExploit, Third Party Advisory
https://wiki.zimbra.com/wiki/Security_CenterRelease Notes, Vendor Advisory
https://wiki.zimbra.com/wiki/Zimbra_Releases/8.8.15/P23Release Notes, Vendor Advisory
https://wiki.zimbra.com/wiki/Zimbra_Releases/9.0.0/P16Release Notes, Vendor Advisory
https://wiki.zimbra.com/wiki/Zimbra_Security_AdvisoriesVendor Advisory

Timeline

Published: Jul 2, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-35209?

How severe is CVE-2021-35209?

CVE-2021-35209 has a CVSS score of 9.8/10 (CRITICAL severity). The EPSS model estimates a 2.97% probability of exploitation in the next 30 days.

How do I fix CVE-2021-35209?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-35209?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST