What’s Vulnerable › Vulnerabilities

CVE-2026-75595: Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext

• Severity: critical (CVSS 9.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-19 · updated 2026-10-26
• Weakness: CWE-754

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.

Affected products

• netty: before 4.1.137; from 4.2.0 before 4.2.17 — fixed in 4.1.137, 4.2.17

How to fix

Upgrade to a fixed version: netty 4.1.137, 4.2.17. Check the vendor advisory for mitigations.

References

• https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7
• https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961
• https://github.com/netty/netty/pull/17213
• https://github.com/netty/netty/pull/17217
• https://github.com/netty/netty/releases/tag/netty-4.1.137.Final
• https://github.com/netty/netty/releases/tag/netty-4.2.17.Final
• https://github.com/netty/netty/security/advisories/GHSA-c4c3-7fpv-j4q5

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