Summary
Netty: [codec-haproxy] Signed-Byte Sentinel Collision in HAProxyMessageDecoder Leads to Unbounded Memory Exhaustion
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.
Impact
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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io.netty:netty-codec-haproxy to 4.2.16.Final or later; io.netty:netty-codec-haproxy to 4.1.136.Final or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-55851? CVE-2026-55851 is a high-severity uncontrolled resource consumption vulnerability in io.netty:netty-codec-haproxy (maven), affecting versions >= 4.2.0.Final, <= 4.2.15.Final. It is fixed in 4.2.16.Final, 4.1.136.Final. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- Which versions of io.netty:netty-codec-haproxy are affected by CVE-2026-55851? io.netty:netty-codec-haproxy (maven) versions >= 4.2.0.Final, <= 4.2.15.Final is affected.
- Is there a fix for CVE-2026-55851? Yes. CVE-2026-55851 is fixed in 4.2.16.Final, 4.1.136.Final. Upgrade to this version or later.
- Is CVE-2026-55851 exploitable, and should I be worried? Whether CVE-2026-55851 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-55851 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-55851?
- Upgrade
io.netty:netty-codec-haproxyto 4.2.16.Final or later - Upgrade
io.netty:netty-codec-haproxyto 4.1.136.Final or later
- Upgrade