CVE-2021-39240Public exploitNetwork attack vector

Information disclosure via malformed HTTP/2 URI fields

Published Aug 17, 2021 · Updated Aug 4, 2024

Improper URI validation 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 read restricted data. HAProxy concatenates unvalidated HTTP/2 :scheme, :authority, and :path fields into an absolute URI after deriving its internal Host header, allowing the recomposed authority or URL prefix to diverge from the value used by routing rules. Exploitation requires HTTP/2 on both the client-facing and backend connections and host-based routing, after which a crafted request reaches unintended backend content without authentication.

CVSS severity7.5
High
EPSS probability2.32%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Improper URI validation 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 read restricted data. HAProxy concatenates unvalidated HTTP/2 :scheme, :authority, and :path fields into an absolute URI after deriving its internal Host header, allowing the recomposed authority or URL prefix to diverge from the value used by routing rules. Exploitation requires HTTP/2 on both the client-facing and backend connections and host-based routing, after which a crafted request reaches unintended backend content without authentication.

The record

CVE
CVE-2021-39240
Published
Aug 17, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-20, 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

Compare your product and version with the public record. A matching version still requires validation against your environment.

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

7.5Red HatCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
7.5CanonicalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/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.

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
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-20Improper Input Validation
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

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo