CVE-2021-4047Network attack vector

Backend routing bypass via conflicting Host authority

Published Apr 11, 2022 · Updated Aug 3, 2024

Improper HTTP/2 request handling in Red Hat OpenShift Container Platform 4.9 allows remote attackers to alter backend Host routing. HAProxy preserves a conflicting Host header instead of giving the HTTP/2 :authority pseudo-header precedence, so successive HTTP/2 hops can derive different backend host values. Exploitation requires an HTTP/2-to-HTTP/2 proxy chain and a crafted unauthenticated request; the mismatch can bypass proxy-layer hostname filtering and route traffic to an unintended backend.

CVSS severity7.5
High
EPSS probability0.93%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper HTTP/2 request handling in Red Hat OpenShift Container Platform 4.9 allows remote attackers to alter backend Host routing. HAProxy preserves a conflicting Host header instead of giving the HTTP/2 :authority pseudo-header precedence, so successive HTTP/2 hops can derive different backend host values. Exploitation requires an HTTP/2-to-HTTP/2 proxy chain and a crafted unauthenticated request; the mismatch can bypass proxy-layer hostname filtering and route traffic to an unintended backend.

The record

CVE
CVE-2021-4047
Published
Apr 11, 2022
Updated
Aug 3, 2024
Vendor
Red Hat
Product
Red Hat OpenShift Container Platform
Classifications
CWE-20, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 11, 2022CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 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?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=OpenShift 4.9

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Red Hat · Red Hat OpenShift Container Platformversion=OpenShift 4.9

Published CVSS scores

5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:P/A:N
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

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.

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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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-20Improper Input Validation
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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