What’s Vulnerable › Vulnerabilities

CVE-2021-35209

• Severity: critical (CVSS 9.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 3.0% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2021-07-02 · updated 2026-06-17
• Weakness: CWE-918

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).

Affected products

• Zimbra Collaboration: from 8.8 before 8.8.15; 8.8.15, 8.8.15-p1, 8.8.15-p10, 8.8.15-p11, 8.8.15-p12, 8.8.15-p13 (+27) — fixed in 8.8.15

How to fix

Upgrade to a fixed version: Zimbra Collaboration 8.8.15. Check the vendor advisory for mitigations.

References

• https://blog.sonarsource.com/zimbra-webmail-compromise-via-email
• https://wiki.zimbra.com/wiki/Security_Center
• https://wiki.zimbra.com/wiki/Zimbra_Releases/8.8.15/P23
• https://wiki.zimbra.com/wiki/Zimbra_Releases/9.0.0/P16
• https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories
• https://blog.sonarsource.com/zimbra-webmail-compromise-via-email
• https://wiki.zimbra.com/wiki/Security_Center
• https://wiki.zimbra.com/wiki/Zimbra_Releases/8.8.15/P23
• https://wiki.zimbra.com/wiki/Zimbra_Releases/9.0.0/P16
• https://wiki.zimbra.com/wiki/Zimbra_Security_Advisories

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