CVE-2026-55831 Netty SPDY SETTINGS frame count materializes unbounded settings map
Remote attackers can cause heap exhaustion and service disruption by sending large SPDY SETTINGS frames to applications using the Netty library.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55831?
CVE-2026-55831 is a high-severity denial of service vulnerability in Netty Netty, published on 20 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. Prior to 4.1.136.Final and 4.2.16.Final, Netty's SPDY SETTINGS decoder accepts a peer-declared SETTINGS entry count up to the 24-bit frame-length limit and materializes every unique setting ID in `DefaultSpdySettingsFrame`, allowing a remote SPDY/3.1 peer to send a syntactically valid roughly 2 MiB SETTINGS frame that creates 262144 map entries and amplifies network input into heap growth and ordered-map insertion work. This issue is fixed in versions 4.1.136.Final and 4.2.16.Final.
Which versions are affected by CVE-2026-55831?
4 open source packages ship an affected version, including io.netty:netty-codec-http. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
io.netty:netty-codec-http Maven | From 4.1.0.Final | 4.1.136.Final |
io.netty:netty-codec-http Maven | From 4.1.0.Final | 4.1.136.Final |
io.netty:netty-codec-http Maven | From 4.2.0.Final | 4.2.16.Final |
io.netty:netty-codec-http Maven | From 4.2.0.Final | 4.2.16.Final |
Products include Netty Netty.
How do I fix CVE-2026-55831?
Upgrade io.netty:netty-codec-http 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-2026-55831 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55831 exists.
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-2026-55851 Netty codec-haproxy: Signed-Byte Sentinel Collision in HAProxyMessageDecoder Leads to Unbounded Memory Exhaustion
- CVE-2026-56745 Netty SpdyHttpDecoder: ByteBuf Reference Leak on RST_STREAM Leads to Native Memory Exhaustion
Sources
Also known as CLEANSTART-2026-ET03364, CLEANSTART-2026-KQ89432, CLEANSTART-2026-LF32209, CLEANSTART-2026-LW60840, CLEANSTART-2026-TM34395, DEBIAN-CVE-2026-55831 and 109 more.