CVE-2026-33555Public exploitNetwork attack vector

Request smuggling via unchecked HTTP/3 body length

Published Apr 13, 2026 · Updated Apr 13, 2026

HTTP request smuggling in HAProxy 2.6 through 3.3.5 allows remote attackers to desynchronize backend requests via HTTP/3. The standalone-FIN path calls qcs_http_handle_standalone_fin() without first invoking h3_check_body_size(), so a zero-length FIN can end a body whose Content-Length promises more bytes. A public demonstration requires a QUIC-enabled build, an HTTP/3 frontend, the non-default http-reuse always setting, and a backend that responds early and drains the missing body, causing another user's request to be consumed as body data.

CVSS severity4.0
Medium
EPSS probability0.30%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP request smuggling in HAProxy 2.6 through 3.3.5 allows remote attackers to desynchronize backend requests via HTTP/3. The standalone-FIN path calls qcs_http_handle_standalone_fin() without first invoking h3_check_body_size(), so a zero-length FIN can end a body whose Content-Length promises more bytes. A public demonstration requires a QUIC-enabled build, an HTTP/3 frontend, the non-default http-reuse always setting, and a backend that responds early and drains the missing body, causing another user's request to be consumed as body data.

The record

CVE
CVE-2026-33555
Published
Apr 13, 2026
Updated
Apr 13, 2026
Vendor
HAProxy Technologies
Product
HAProxy
Classifications
CWE-130, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 13, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 13, 2026Record 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=2.6 <3.3.6

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

HAProxy Technologies · HAProxyversion=2.6 <3.3.6

Published CVSS scores

4.0MITRECVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N
5.8NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/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-130Improper Handling of Length Parameter Inconsistency
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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