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.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Java / Kotlin (Maven) | io.netty:netty-codec-haproxy | >= 4.2.0.Final, < 4.2.16.Final, >= 4.1.0.Final, < 4.1.136.Final | 4.2.16.Final, 4.1.136.Final |
References
- https://github.com/netty/netty/security/advisories/GHSA-q6cq-mhr2-jmr5
- https://nvd.nist.gov/vuln/detail/CVE-2026-55851
- https://github.com/netty/netty/commit/5b68c61f37aa4a3045cba624cbea239655c9003b
- https://github.com/netty/netty/commit/bb2ff68a1fb71cb4b0eb9a9e17b66c52aff680c6
- https://github.com/netty/netty
- https://github.com/netty/netty/releases/tag/netty-4.1.136.Final
- https://github.com/netty/netty/releases/tag/netty-4.2.16.Final
- CVSS vector
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- Weakness (CWE)
CWE-400- Platform
- Java / Kotlin (Maven)
- Package
io.netty:netty-codec-haproxy- Disclosed
- 22.07.2026
- Updated
- 11.10.2026 13:20
Sources and licenses
OSV / GitHub Advisory Database - GHSA-q6cq-mhr2-jmr5 - CC BY 4.0
Vulnerability history: io.netty:netty-codec-haproxy
All published vulnerabilities of this package (Java / Kotlin (Maven)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 5
- First → latest
- 12.2022 07.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 3
- Medium 2
Disclosures per year
- 2022: 1
- 2023: 0
- 2024: 0
- 2025: 0
- 2026: 4
-
Netty: [codec-haproxy] Signed-Byte Sentinel Collision in HAProxyMessageDecoder Leads to Unbounded Memory Exhaustion this entry
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