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CVE Record

CVE-2021-35209: An issue was discovered in ProxyServlet.java in the /proxy servlet in Zimbra Collaboration Suite 8.8 before...

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).

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

CVE-2021-35209 affects Zimbra Collaboration Suite webmail proxy behavior. A request header can override the normal destination host check, allowing proxied requests to bypass Zimbra's allowed-domain setting. For organizations exposing Zimbra webmail, this is a meaningful perimeter risk even though the source bundle does not provide a CVSS score.

Executive priority

Prioritize remediation for internet-facing Zimbra systems. The flaw weakens a boundary control in a mail platform, and mail systems often carry sensitive data and trusted user sessions. Patch confirmation should be tracked as a security operations item.

Technical view

In ProxyServlet.java for the /proxy servlet, X-Host overwrites Host in proxied requests. The X-Host value is not validated against zimbraProxyAllowedDomains, bypassing the intended whitelist. Affected versions are Zimbra 8.8 before 8.8.15 Patch 23 and 9.x before 9.0.0 Patch 16.

Likely exposure

Exposure is most likely where affected Zimbra Collaboration Suite webmail is reachable by untrusted users, especially internet-facing deployments running before 8.8.15 Patch 23 or 9.0.0 Patch 16.

Exploitation context

The bundle includes technical discussion from SonarSource and vendor patch references. It does not identify CISA KEV listing or provide sourced evidence of active exploitation for this CVE, so active exploitation should not be asserted from these sources alone.

Researcher notes

Evidence supports a host whitelist bypass in Zimbra's /proxy servlet through X-Host handling. The source bundle names fixed patch levels but does not include CVSS, CWE, exploit maturity, or KEV status. Treat impact analysis as source-limited and validate against local exposure.

Mitigation direction

  • Upgrade Zimbra 8.8.15 deployments to Patch 23 or later.
  • Upgrade Zimbra 9.0 deployments to Patch 16 or later.
  • Review Zimbra security advisories for current vendor guidance.
  • Limit external access to Zimbra webmail where operationally feasible.
  • Monitor for unusual /proxy requests involving X-Host headers.

Validation and detection

  • Inventory all Zimbra Collaboration Suite versions and patch levels.
  • Confirm exposed systems are not below 8.8.15 Patch 23 or 9.0.0 Patch 16.
  • Review configuration for zimbraProxyAllowedDomains intent and proxy exposure.
  • Check web/proxy logs for suspicious X-Host usage.
  • Validate remediation against Zimbra release notes and security advisories.
Prepared
Confidence
high
Sources
7

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
0Timeline events
0ADP providers
6Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/an/an/aListed
Weakness

CWE details

No CWE listed

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