Cyber Resilience

CVE-2026-33555

Haproxy 2.6.0 – 3.3.6

Public PoC
Published
13 April 2026
Modified
29 June 2026
Patch / advisory
View advisory
CVSS Score v3.1 4.0
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:L/A:N
EPSS Score 0.0030 22th percentile
Risk Priority 33 floored blend · peak EPSS

CVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.

Summary

CVE-2026-33555 is a medium-severity Improper Handling of Length Parameter Inconsistency (CWE-130) vulnerability in Haproxy Haproxy. Its CVSS base score is 4.0 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploitation for Privilege Escalation (T1068); ranked at the 22th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.

The strongest mitigations our analysis identified map to SA-11 (Developer Testing and Evaluation) and SI-10 (Information Input Validation) — see the control section below for these in your framework.

EU & UK References

Vulnerability Data

An issue was discovered in HAProxy before 3.3.6. The HTTP/3 parser does not check that the received body length matches a previously announced content-length when the stream is closed via a frame with an empty payload. This can cause desynchronization…

more

issues with the backend server and could be used for request smuggling. The earliest affected version is 2.6.

CWE(s)
CWE-130

Related Threats

MITRE ATT&CK Enterprise Techniques

T1068 Exploitation for Privilege Escalation Privilege Escalation
Adversaries may exploit software vulnerabilities in an attempt to elevate privileges.
attack.mitre.org →
T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
T1210 Exploitation of Remote Services Lateral Movement
Adversaries may exploit remote services to gain unauthorized access to internal systems once inside of a network.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2026-55203Same product: Haproxy Haproxy
CVE-2026-26081Same product: Haproxy Haproxy
CVE-2023-45539Same product: Haproxy Haproxy
CVE-2023-40225Same product: Haproxy Haproxy
CVE-2023-25950Same product: Haproxy Haproxy
CVE-2026-55204Same product: Haproxy Haproxy
CVE-2023-0836Same product: Haproxy Haproxy
CVE-2024-45506Same product: Haproxy Haproxy
CVE-2026-26080Same product: Haproxy Haproxy
CVE-2024-37989Shared CWE-130

Affected Assets

haproxy
haproxy
2.6.0 — 3.3.6

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V3.5.5
  • V4.2.1
  • V4.2.2

Mitigating Controls (NIST 800-53 r5) AI

SA-11 Developer Testing and Evaluation good match

Developer testing and evaluation can discover length-parameter inconsistencies through targeted parsing tests.

SI-10 Information Input Validation good match

Input validation enforces correct length-to-data consistency checks on parsed messages before processing.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly enforce length validation and input sanitization during coding and review.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing in development can detect length-inconsistency vulnerabilities before deployment.

A.8.25 Secure development life cycle partial match
prevents

Secure development lifecycle mandates input validation and length checks that directly address inconsistent length fields.

A.8.26 Application security requirements partial match
prevents

Application security requirements include explicit rules for handling message lengths and data structures.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure architecture principles require robust parsing and bounds checking to prevent length-related flaws.

A.8.28 Secure coding partial match
prevents

Secure coding standards directly prohibit improper length handling and enforce defensive parsing practices.

References