How severe is CVE-2026-55851?

No CVSS score has been assigned to CVE-2026-55851 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-55851?

CVE-2026-55851 affects the following packages: io.netty:netty-codec-haproxy (Maven), io.netty:netty-codec-haproxy (Maven). Ecosystems affected: Maven.

How do I fix CVE-2026-55851?

Update io.netty:netty-codec-haproxy to 4.2.16.Final or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55851 is resolved across your whole dependency graph.

How do I detect CVE-2026-55851 in my Maven dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.netty:netty-codec-haproxy, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-55851 if there is no patch (or I can't update yet)?

Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Is CVE-2026-55851 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-55851 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-55851?

CVE-2026-55851 has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 49th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-55851?

CVE-2026-55851 is classified as Uncontrolled Resource Consumption (CWE-400). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-55851 published, and has it been updated?

CVE-2026-55851 was published on July 21, 2026 and was last updated on August 12, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕ Maven

Not in CISA KEV

CVE-2026-55851 — netty-codec-haproxy

Fix: netty/netty@5b68c61

CVE-2026-55851 is a Uncontrolled Resource Consumption vulnerability in io.netty:netty-codec-haproxy. A fix is available for io.netty:netty-codec-haproxy — see the affected versions and patch details below.

Netty codec-haproxy: Signed-Byte Sentinel Collision in HAProxyMessageDecoder Leads to Unbounded Memory Exhaustion

Also known asGHSA-q6cq-mhr2-jmr5

Published

Jul 21, 2026

Updated

Aug 12, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Oct 17, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-55851.

EPSS Exploitation Probability

via FIRST.org ↗

0.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs49th percentile — riskier than 49% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

2 pkgs affected

☕io.netty:netty-codec-haproxy☕io.netty:netty-codec-haproxy

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

The HAProxyMessageDecoder in netty's codec-haproxy module performs protocol version detection by reading the 13th byte of the inbound stream as a signed Java byte and widening it to int without masking. When an attacker sends a PROXY protocol v2 binary prefix (0D 0A 0D 0A 00 0D 0A 51 55 49 54 0A) followed by version byte 0xFF, the sign extension produces -1, which collides with the decoder's "need more data" sentinel value. This collision traps the decoder in a version-detection loop where it perpetually requests more data without consuming any bytes, never instantiates the HeaderExtractor that enforces header size limits, and causes ByteToMessageDecoder to accumulate all subsequent inbound bytes into an unbounded cumulation buffer until the JVM exhausts its direct memory allocation.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`io.netty:netty-codec-haproxy`	≥ 4.2.0.Final&&< 4.2.16.Final	4.2.16.Final`io.netty:netty-codec-haproxy:4.2.16.Final`
☕Maven	`io.netty:netty-codec-haproxy`	≥ 4.1.0.Final&&< 4.1.136.Final	4.1.136.Final`io.netty:netty-codec-haproxy:4.1.136.Final`

Affected Products

1 product · 2 configurations

Application

nettynetty

≥ 4.2.0 && < 4.2.16

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for io.netty:netty-codec-haproxy, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update io.netty:netty-codec-haproxy to 4.2.16.Final or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55851 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

This is an Important denial-of-service vulnerability in the Netty `codec-haproxy` module. A remote attacker can send a specially crafted PROXY protocol v2 message, causing unbounded memory accumulation and exhausting direct memory, leading to service unavailability. This impact is significant for applications that…

Product	Fixed in	Advisory
Cryostat 4 on RHEL 9	`cryostat/cryostat-reports-rhel9:4.2.0-14`	RHSA-2026:68333
Red Hat AMQ Broker 7.14.1	`netty-codec-haproxy`	RHSA-2026:66488
Red Hat Build of Apache Camel 3.33.2.SP3	`io.netty/netty-codec-haproxy:4.1.136.Final-redhat-00001`	RHSA-2026:48118
Red Hat build of Apache Camel 4.18.3 for Spring Boot 3.5.16	`netty-codec-haproxy`	RHSA-2026:54622
Red Hat build of Quarkus 3.27.4.SP3	`netty-codec-haproxy`	RHSA-2026:47189
Red Hat build of Quarkus 3.33.2.SP3	`netty-codec-haproxy`	RHSA-2026:47172
Red Hat Data Grid 8.6.3	`netty-codec-haproxy`	RHSA-2026:69296
Red Hat OpenShift AI 2.25	`rhoai/odh-trustyai-service-rhel9:1788198681`	RHSA-2026:65126

Frequently Asked Questions

The `HAProxyMessageDecoder` in netty's `codec-haproxy` module performs protocol version detection by reading the 13th byte of the inbound stream as a signed Java `byte` and widening it to `int` without masking. When an attacker sends a PROXY protocol v2 binary prefix (`0D 0A 0D 0A 00 0D 0A 51 55 49 54 0A`) followed by version byte `0xFF`, the sign extension produces `-1`, which collides with the decoder's "need more data" sentinel value. This collision traps the decoder in a version-detection loop where it perpetually requests more data without consuming any bytes, never instantiates the `Head