HOMEVULNERABILITIESCVE-2021-35209
CRITICAL

CVE-2021-35209

CWE-918Published: July 2, 2021· Updated: Jun 17, 2026

9.8
CVSS v3.1

Official 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).

NVD Source

Technical Analysis

CVE-2021-35209 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.

The vulnerability requires no privileges and no user interaction, making it a prime target for automated exploitation campaigns and worm-like propagation.

A successful exploit results in complete confidentiality breach (data exposure), full integrity compromise (data manipulation), availability disruption (denial of service), with a CVSS base score of 9.8.

CVSS v3.1 Vector Breakdown

Exploitability
Attack VectorNetwork
Attack ComplexityLow
Privileges Req.None
User InteractionNone
ScopeUnchanged
Impact
ConfidentialityHigh
IntegrityHigh
AvailabilityHigh
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Affected Vendors & Products

zimbra1 product(s)
collaboration
Source: NVD CPE · 3 total CPE entries

Exploit & PoC Resources

NO KNOWN EXPLOITNo public exploit confirmed at this time
Exploit-DB
Offensive Security exploit database
GitHub PoC Search
Community PoC repositories
Rapid7 / Metasploit
Metasploit module search
PacketStorm
Security advisories & exploits
NVD Detail
Official NIST entry
MITRE CVE
CVE Program entry
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Official Patches & Advisories

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

All References (10)

→
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

Quick Facts

CVE IDCVE-2021-35209
CVSS Score9.8 / 10
SeverityCRITICAL
WeaknessCWE-918
CISA KEVNo
Affected1 vendor(s)
PublishedJul 2, 2021

Related CVEs (CWE-918)

CVE-2026-35431
10.0
CVE-2026-26150
10.0
CVE-2023-39967
10.0
CVE-2025-2828
10.0
CVE-2026-2053
10.0
CVE-2026-32169
10.0

Recommended Actions

  • →Apply vendor patches immediately
  • →Monitor CVE-2021-35209 in threat intel feeds
  • →Review IDS/IPS signatures for exploitation attempts
Data sourced from NVD (NIST), CISA KEV, and EPSS (FIRST). Analysis generated by CTIWATCH.COM. CVE data is provided under the NVD usage policy.
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