CVE-2026-55851Network attack vector

Remote memory exhaustion via signed-byte sentinel collision

Published Jul 21, 2026 · Updated Jul 21, 2026

Resource exhaustion in affected Netty codec-haproxy releases allows remote attackers to crash JVM services with a malformed PROXY v2 header. HAProxyMessageDecoder widens the thirteenth byte as a signed value, so 0xFF becomes -1 and collides with its need-more-data sentinel, bypassing header extraction while ByteToMessageDecoder retains subsequent bytes in an unbounded cumulation buffer. The issue requires codec-haproxy on an untrusted channel and affects 4.1.0.Final through 4.1.135.Final and 4.2.0.Final through 4.2.15.Final; sustained input exhausts direct memory and terminates the JVM process.

CVSS severity8.7
High
EPSS probability0.61%
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

Resource exhaustion in affected Netty codec-haproxy releases allows remote attackers to crash JVM services with a malformed PROXY v2 header. HAProxyMessageDecoder widens the thirteenth byte as a signed value, so 0xFF becomes -1 and collides with its need-more-data sentinel, bypassing header extraction while ByteToMessageDecoder retains subsequent bytes in an unbounded cumulation buffer. The issue requires codec-haproxy on an untrusted channel and affects 4.1.0.Final through 4.1.135.Final and 4.2.0.Final through 4.2.15.Final; sustained input exhausts direct memory and terminates the JVM process.

The record

CVE
CVE-2026-55851
Published
Jul 21, 2026
Updated
Jul 21, 2026
Vendor
The Netty Project
Product
netty-codec-haproxy
Classifications
CWE-400, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 21, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 21, 2026Record 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?

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

  1. Affected versionversion=>= 4.1.0.Final, < 4.1.136.Final
  2. Affected versionversion=>= 4.2.0.Final, < 4.2.16.Final

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The Netty Project · netty-codec-haproxyversion=>= 4.1.0.Final, < 4.1.136.Final; version=>= 4.2.0.Final, < 4.2.16.Final

Published CVSS scores

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
8.7GitHub, Inc.CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA: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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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