CVE-2021-35209Public exploitNetwork attack vector

Internal response disclosure via unchecked X-Host override

Published Jul 2, 2021 · Updated Aug 4, 2024

SSRF in Zimbra Collaboration Suite 8.8 before Patch 23 and 9.0 before Patch 16 allows remote authenticated users to read internal service responses. ProxyServlet.java copies the unvalidated X-Host request header into the outbound Host header, letting an attacker-controlled redirect steer the proxy to arbitrary URLs. Any organizational user role can reach the servlet; cloud metadata credentials and localhost services are exposed when reachable from the server.

CVSS severity9.8
Critical
EPSS probability3.00%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

SSRF in Zimbra Collaboration Suite 8.8 before Patch 23 and 9.0 before Patch 16 allows remote authenticated users to read internal service responses. ProxyServlet.java copies the unvalidated X-Host request header into the outbound Host header, letting an attacker-controlled redirect steer the proxy to arbitrary URLs. Any organizational user role can reach the servlet; cloud metadata credentials and localhost services are exposed when reachable from the server.

The record

CVE
CVE-2021-35209
Published
Jul 2, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-918, T1190, T1552.005
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Jul 2, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8NIST National Vulnerability DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NIST National Vulnerability DatabaseAV:N/AC:L/Au:N/C:P/I:P/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-918Server-Side Request Forgery (SSRF)
T1190Exploit Public-Facing Application
T1552.005Cloud Instance Metadata API

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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