CVE-2026-55851: Netty uncontrolled resource consumption in HAProxyMessageDecoder

CVE-2026-55851 · Severity: high · CVSS 4 · Published 2026-07-21

Netty is a widely used networking framework for Java applications. A vulnerability in its HAProxy protocol module allows a remote attacker to crash a server by sending a specially crafted data packet. This causes the server to consume all available memory, leading to a denial-of-service (DoS) condition that disrupts operations and availability.

Technologies: io.netty:netty-codec-haproxy (Maven). Vendors: Maven.

Quick answers

Is CVE-2026-55851 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-55851 being exploited in the wild.
How severe is CVE-2026-55851?
High severity, with a CVSS score of 4, published 21 July 2026.
Which products and versions are affected?
io.netty:netty-codec-haproxy (Maven) ECOSYSTEM:introduced=4.2.0.Final,fixed=4.2.16.Final.
Is there a fix for CVE-2026-55851?
Yes. io.netty:netty-codec-haproxy (Maven): upgrade to 4.2.16.Final or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-55851 attacked?
It is attacked over the network, in a software library.
Am I affected by CVE-2026-55851?
Check whether you run io.netty:netty-codec-haproxy (Maven) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

  • io.netty:netty-codec-haproxy (Maven): upgrade to 4.2.16.Final or later
    # set io.netty:netty-codec-haproxy to 4.2.16.Final in your pom.xml or build.gradle

Technical details

The HAProxyMessageDecoder in Netty's codec-haproxy module incorrectly handles protocol version detection by reading the 13th byte as a signed Java byte and widening it to an integer without masking. An attacker can send a PROXY protocol v2 binary prefix followed by a version byte of 0xFF, which sign-extends to -1. This value collides with the decoder's internal 'need more data' sentinel. This collision traps the decoder in a loop where it continuously requests more data without consuming bytes or enforcing header size limits, leading to unbounded accumulation in the cumulation buffer and JVM direct memory exhaustion. The issue is fixed in versions 4.1.136.Final and 4.2.16.Final.

Affected products

  • Netty netty-codec-haproxy >= 4.1.0.Final, <= 4.1.135.Final
  • Netty netty-codec-haproxy >= 4.2.0.Final, <= 4.2.15.Final

Identifiers

  • CVE-2026-55851
  • GHSA-q6cq-mhr2-jmr5

Timeline

  • 2026-07-14: disclosed: Initial disclosure by maintainers
  • 2026-07-21: advisory: NVD publication
  • 2026-07-22: advisory: GitHub Advisory reviewed

References

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