CVE-2026-55851 Netty codec-haproxy: Signed-Byte Sentinel Collision in HAProxyMessageDecoder Leads to Unbounded Memory Exhaustion
A signed-byte collision in Netty's HAProxy decoder causes unbounded memory accumulation, leading to denial of service via direct memory exhaustion.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55851?
CVE-2026-55851 is a high-severity denial of service vulnerability in Netty Netty, published on 21 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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.
Weakness: CWE-400
Which versions are affected by CVE-2026-55851?
4 open source packages ship an affected version, including io.netty:netty-codec-haproxy. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
io.netty:netty-codec-haproxy Maven | From 4.1.0.Final | 4.1.136.Final |
io.netty:netty-codec-haproxy Maven | From 4.1.0.Final | 4.1.136.Final |
io.netty:netty-codec-haproxy Maven | From 4.2.0.Final | 4.2.16.Final |
io.netty:netty-codec-haproxy Maven | From 4.2.0.Final | 4.2.16.Final |
Products include Netty Netty.
How do I fix CVE-2026-55851?
Upgrade io.netty:netty-codec-haproxy to a fixed version for your release line: 4.1.136.Final or 4.2.16.Final.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-55851 being exploited?
There is no known exploitation of CVE-2026-55851 in the wild and no public exploit code.
Other vulnerabilities in Netty Netty
- CVE-2026-75595 Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext
- CVE-2026-75596 Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing
- CVE-2026-59901 Netty Bzip2Decoder: Infinite Loop in RLE State Machine Leads to Event-Loop Thread Hang
- CVE-2026-56745 Netty SpdyHttpDecoder: ByteBuf Reference Leak on RST_STREAM Leads to Native Memory Exhaustion
- CVE-2026-45674 Netty Vulnerable to DNS Cache Poisoning via Missing Bailiwick Checks in CNAME Records
Sources
Also known as CLEANSTART-2026-CY63750, DEBIAN-CVE-2026-55851, ECHO-1bfe-a736-df50, GHSA-q6cq-mhr2-jmr5, MINI-247v-77rf-66vc, MINI-25f6-jpfg-626c and 37 more.