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Netty CVE-2026-55851

| EUVDEUVD-2026-46438 HIGH
Uncontrolled Resource Consumption (CWE-400)
2026-07-21 GitHub_M GHSA-q6cq-mhr2-jmr5
Denial Of Service Java Red Hat Netty
8.7
CVSS 4.0 · Vendor: GitHub_M
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Severity by source

Vendor (GitHub_M) PRIMARY
8.7 HIGH
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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
vuln.today AI
7.5 HIGH

Network-reachable, unauthenticated, low-complexity trigger causing memory-exhaustion DoS; availability-only impact, so C:N/I:N/A:H with PR:N and UI:N.

3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
SUSE
7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Red Hat
7.5 HIGH
qualitative

Primary rating from Vendor (GitHub_M).

CVSS VectorVendor: GitHub_M

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
X

Lifecycle Timeline

4
Patch available
Jul 21, 2026 - 23:36 EUVD
Source Code Evidence Fetched
Jul 21, 2026 - 22:05 vuln.today
Analysis Generated
Jul 21, 2026 - 22:05 vuln.today
CVE Published
Jul 21, 2026 - 21:22 cve.org
HIGH 8.7

Blast Radius

ecosystem impact
† from your stack dependencies † transitive graph · vuln.today resolves 4-path depth
  • 75 maven packages depend on io.netty:netty-codec-haproxy (3 direct, 72 indirect)

Ecosystem-wide dependent count for version 4.2.0.Final.

DescriptionCVE.org

Netty is a network application framework for development of protocol servers and clients. In versions 4.2.0.Final up to (but not including) 4.2.16.Final, and 4.1.0.Final up to (but not including) 4.1.135, the HAProxyMessageDecoder in Netty's codec-haproxy module performs protocol version detection by reading the 13th byte as a signed Java byte and widening it to int without masking; a PROXY protocol v2 binary prefix followed by version byte 0xFF sign-extends to -1, collides with the decoder's need-more-data sentinel, and causes ByteToMessageDecoder to accumulate inbound bytes in an unbounded cumulation buffer until direct memory is exhausted. This issue is fixed in versions 4.1.136.Final and 4.2.16.Final.

Articles & Coverage 1

News 6 New Java Vulnerabilities - 1 Critical, 5 High Severity Jul 22, 2026

AnalysisAI

Remote denial of service in the Netty networking framework (codec-haproxy module) versions 4.1.0.Final through 4.1.135 and 4.2.0.Final through 4.2.15.Final lets unauthenticated attackers exhaust JVM direct memory. The HAProxyMessageDecoder mis-parses a PROXY protocol v2 header whose version byte is 0xFF, sign-extending it to -1, which collides with the decoder's 'need more data' sentinel, so ByteToMessageDecoder accumulates inbound bytes in an unbounded cumulation buffer until the process runs out of direct memory. No public exploit identified at time of analysis; the CVSS 4.0 score of 8.7 reflects high availability impact with no confidentiality or integrity effect.

Technical ContextAI

Netty is a widely used asynchronous, event-driven Java network application framework that underpins many servers, proxies, and middleware libraries. The flaw is in the optional codec-haproxy module, which implements the HAProxy PROXY protocol used to preserve original client connection metadata across load balancers. HAProxyMessageDecoder detects the protocol version by reading the 13th byte of a PROXY v2 binary header. Java bytes are signed, and the decoder widens this byte to int without masking with 0xFF, so a value of 0xFF becomes -1 instead of 255. That -1 collides with the internal sentinel ByteToMessageDecoder uses to signal 'not enough data yet', so the decoder never makes progress and keeps buffering every subsequent inbound byte in its cumulation buffer. This is a classic CWE-400 Uncontrolled Resource Consumption: unbounded off-heap/direct memory growth driven by attacker-supplied input.

RemediationAI

Vendor-released patch: upgrade to Netty 4.1.136.Final (https://github.com/netty/netty/releases/tag/netty-4.1.136.Final) or 4.2.16.Final (https://github.com/netty/netty/releases/tag/netty-4.2.16.Final) per advisory GHSA-q6cq-mhr2-jmr5. Because Netty is usually a transitive dependency, also rebuild and repin downstream frameworks that bundle it. If you cannot patch immediately, the most direct compensating control is to stop exposing HAProxyMessageDecoder to untrusted input: remove the codec-haproxy handler from pipelines that face untrusted networks, or restrict PROXY-protocol acceptance to trusted upstream load balancers only via network ACLs or firewall rules limiting who can speak PROXY protocol to the listener. As a further guard, cap off-heap usage with -Dio.netty.maxDirectMemory and set conservative per-connection read limits so the cumulation buffer trips a bound instead of exhausting the JVM; the trade-off is that legitimate large frames may be rejected and connections closed. These controls reduce blast radius but do not fix the parsing bug, so upgrading remains the priority.

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Vendor StatusVendor

SUSE

Severity: Important
Product Status
SUSE Linux Enterprise Desktop 15 SP7 Not-Affected
SUSE Linux Enterprise Desktop 15 SP7 Not-Affected
SUSE Linux Enterprise High Performance Computing 15 SP7 Not-Affected
SUSE Linux Enterprise High Performance Computing 15 SP7 Not-Affected
SUSE Linux Enterprise Module for Development Tools 15 SP7 Not-Affected

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CVE-2026-55851 vulnerability details – vuln.today
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