What is CVE-2026-75595?

CVE-2026-75595 is a critical-severity security vulnerability in io.netty:netty-handler (maven), affecting versions >= 4.2.0.Final, <= 4.2.16.Final. It is fixed in 4.2.17.Final, 4.1.137.Final.

Which versions of io.netty:netty-handler are affected by CVE-2026-75595?

io.netty:netty-handler (maven) versions >= 4.2.0.Final, <= 4.2.16.Final is affected.

Is there a fix for CVE-2026-75595?

Yes. CVE-2026-75595 is fixed in 4.2.17.Final, 4.1.137.Final. Upgrade to this version or later.

Is CVE-2026-75595 exploitable, and should I be worried?

Whether CVE-2026-75595 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

What actually determines whether CVE-2026-75595 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

CVE-2026-75595 in io.netty:netty-handler

Q: Which versions of io.netty:netty-handler are affected by CVE-2026-75595?

io.netty:netty-handler (maven) versions >= 4.2.0.Final, <= 4.2.16.Final is affected.

Q: Is there a fix for CVE-2026-75595?

Yes. CVE-2026-75595 is fixed in 4.2.17.Final, 4.1.137.Final. Upgrade to this version or later.

Q: Is CVE-2026-75595 exploitable, and should I be worried?

Whether CVE-2026-75595 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

Q: What actually determines whether CVE-2026-75595 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

Q: How do I fix CVE-2026-75595?

Upgrade: io.netty:netty-handler to 4.2.17.Final or later, io.netty:netty-handler to 4.1.137.Final or later.

Summary

Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext

A fragmented TLS ClientHello whose handshake header spans multiple records makes Netty silently fall back to the default SslContext; where per-SNI selection is the sole mTLS gate, an unauthenticated attacker can bypass the route's mTLS requirement.

Details

In io.netty.handler.ssl.SslClientHelloHandler#decode the guard that should wait for the 4-byte handshake header checks the wrong offset - it ignores the 5-byte record header that precedes it - and therefore never fires:

if (handshakeLength == -1) {
    if (readerIndex + 4 > endOffset) {
        // Need more data to read HandshakeType and handshakeLength (4 bytes)
        return;
    }

When the first record's payload is < 4 bytes, handshakeLength = in.getUnsignedMedium(readerIndex + SslUtils.SSL_RECORD_HEADER_LENGTH + 1); leads to IndexOutOfBoundsException . That is caught by the generic catch (Exception) block, which calls select(ctx, null) - this is the default SslContext. Fallback to default on parse failure is a problem when per-SNI selection is the sole mTLS gate.

Impact

SNI routing bypass. Escalates to an unauthenticated mTLS bypass only when:

mTLS is enforced solely via per-SNI SslContext (clientAuth=REQUIRE)
the default/fallback SslContext is permissive (clientAuth=NONE/OPTIONAL)
no secondary peer-certificate verification exists at the application layer.

Affected versions

io.netty:netty-handler (>= 4.2.0.Final, <= 4.2.16.Final) io.netty:netty-handler (<= 4.1.136.Final)

Security releases

io.netty:netty-handler → 4.2.17.Final (maven) io.netty:netty-handler → 4.1.137.Final (maven)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

Upgrade the following packages to resolve this vulnerability:

io.netty:netty-handler to 4.2.17.Final or later; io.netty:netty-handler to 4.1.137.Final or later

Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.

Frequently Asked Questions

What is CVE-2026-75595? CVE-2026-75595 is a critical-severity security vulnerability in io.netty:netty-handler (maven), affecting versions >= 4.2.0.Final, <= 4.2.16.Final. It is fixed in 4.2.17.Final, 4.1.137.Final.
Which versions of io.netty:netty-handler are affected by CVE-2026-75595? io.netty:netty-handler (maven) versions >= 4.2.0.Final, <= 4.2.16.Final is affected.
Is there a fix for CVE-2026-75595? Yes. CVE-2026-75595 is fixed in 4.2.17.Final, 4.1.137.Final. Upgrade to this version or later.
Is CVE-2026-75595 exploitable, and should I be worried? Whether CVE-2026-75595 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
What actually determines whether CVE-2026-75595 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
How do I fix CVE-2026-75595?
- Upgrade io.netty:netty-handler to 4.2.17.Final or later
- Upgrade io.netty:netty-handler to 4.1.137.Final or later