CVE-2021-39242Public exploitNetwork attack vector

Host routing bypass via authority mismatch

Published Aug 17, 2021 · Updated Aug 4, 2024

Host header validation bypass in HAProxy 2.2 before 2.2.16, 2.3 before 2.3.13, and 2.4 before 2.4.3 allows remote attackers to evade routing rules. HAProxy evaluates switching rules against the HTTP Host header, then drops that header when forwarding to an HTTP/2 backend and reconstructs it from a conflicting :authority value. Exploitation requires HTTP/2 on both sides and can route a request under a different host identity than the proxy validated.

CVSS severity7.5
High
EPSS probability2.34%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Host header validation bypass in HAProxy 2.2 before 2.2.16, 2.3 before 2.3.13, and 2.4 before 2.4.3 allows remote attackers to evade routing rules. HAProxy evaluates switching rules against the HTTP Host header, then drops that header when forwarding to an HTTP/2 backend and reconstructs it from a conflicting :authority value. Exploitation requires HTTP/2 on both sides and can route a request under a different host identity than the proxy validated.

The record

CVE
CVE-2021-39242
Published
Aug 17, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-755, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 17, 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 privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

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

Weakness, pattern, technique

CWE-755Improper Handling of Exceptional Conditions
T1190Exploit Public-Facing Application

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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