CVE-2026-75596 Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing
Remote attackers can cause CPU exhaustion and service degradation by sending fragmented TLS ClientHello records to applications using the default SNI handler.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-75596?
CVE-2026-75596 is a high-severity denial of service vulnerability in Netty Netty, published on 19 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Netty is an asynchronous, event-driven network application framework. Prior to 4.1.137.Final and 4.2.17.Final, the default io.netty.handler.ssl.SniHandler constructors use the pre-handshake ClientHello aggregation path in handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java at io.netty.handler.ssl.SslClientHelloHandler#decode, where handshakeBuffer.clear() and writeBytes() recopy all previously received body bytes for every additional TLS record. An unauthenticated remote peer can advertise a large ClientHello and deliver its body in thousands of tiny records, causing quadratic CPU work on the event loop before the TLS handshake completes and degrading TLS handling for other clients. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.
Weakness: CWE-407
Which versions are affected by CVE-2026-75596?
4 open source packages ship an affected version, including io.netty:netty-handler. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
io.netty:netty-handler Maven | All versions | 4.1.137.Final |
io.netty:netty-handler Maven | All versions | 4.1.137.Final |
io.netty:netty-handler Maven | From 4.2.0.Final | 4.2.17.Final |
io.netty:netty-handler Maven | From 4.2.0.Final | 4.2.17.Final |
Products include Netty Netty.
How do I fix CVE-2026-75596?
Upgrade io.netty:netty-handler to a fixed version for your release line: 4.1.137.Final or 4.2.17.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-2026-75596 being exploited?
There is no known exploitation of CVE-2026-75596 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-2026-59901 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
- CVE-2026-45674 Netty Vulnerable to DNS Cache Poisoning via Missing Bailiwick Checks in CNAME Records
Sources
Also known as CLEANSTART-2026-BB16699, CLEANSTART-2026-BL22559, CLEANSTART-2026-BP52877, CLEANSTART-2026-CQ05064, CLEANSTART-2026-EO12643, CLEANSTART-2026-LV23492 and 119 more.