CVE-2026-75595 Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext
An attacker can bypass mutual TLS authentication in Netty by sending fragmented TLS ClientHello messages, accessing protected routes without valid certificates.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-75595?
CVE-2026-75595 is a critical authentication bypass 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.Fina and 4.2.17.Final, io.netty.handler.ssl.SslClientHelloHandler#decode checks the wrong offset before reading the four-byte TLS handshake header, so a ClientHello whose handshake header spans records can cause an IndexOutOfBoundsException and invoke select(ctx, null). This selects the default SslContext instead of the SNI-specific context. In deployments where per-SNI clientAuth=REQUIRE is the sole mutual TLS gate, the default SslContext uses clientAuth=NONE or clientAuth=OPTIONAL, and no application-layer certificate verification exists, an unauthenticated remote attacker can bypass the protected route's mutual TLS requirement. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.
Weakness: CWE-754
Which versions are affected by CVE-2026-75595?
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-2026-75595?
Upgrade io.netty:netty-handler to a fixed version for your release line: 4.1.137.Final or 4.2.17.Final.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-75595 being exploited?
There is no known exploitation of CVE-2026-75595 in the wild and no public exploit code.
Other vulnerabilities in Netty Netty
- 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
- CVE-2026-45674 Netty Vulnerable to DNS Cache Poisoning via Missing Bailiwick Checks in CNAME Records
Sources
Also known as CLEANSTART-2026-FZ84712, CLEANSTART-2026-HW50386, CLEANSTART-2026-HY48105, CLEANSTART-2026-JA68863, CLEANSTART-2026-KP61790, CLEANSTART-2026-PE28143 and 121 more.