CVE-2026-75595: Netty SNI Routing Bypass Vulnerability

Q: What is CVE-2026-75595?

CVE-2026-75595 is a security vulnerability in Netty involving SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext under CWE-754: Improper Check for Unusual or Exceptional Conditions.

Q: Which Netty versions are affected?

Affected versions include releases prior to 4.2.17.Final, 4.1.137.Final across affected packages: io.netty:netty-handler.

Q: What version fixes CVE-2026-75595?

Upgrading to 4.2.17.Final, 4.1.137.Final (or the latest patched release for your respective branch) resolves the vulnerability.

Q: Is netty-handler affected?

Yes, io.netty:netty-handler is affected in releases prior to 4.2.17.Final, 4.1.137.Final. Upgrade your dependency to 4.2.17.Final, 4.1.137.Final or later.

Q: What CWE is associated with CVE-2026-75595?

CVE-2026-75595 is classified under CWE-754: Improper Check for Unusual or Exceptional Conditions (The product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product.).

Q: How can I check Maven dependencies for CVE-2026-75595?

Run the dependency tree command: 'mvn dependency:tree -Dincludes=io.netty'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.

CRITICAL

Advisory Identifier: GHSA-c4c3-7fpv-j4q5

CWE-754 CWE-475

Published 2026-09-08

Advisory Summary

CVE-2026-75595 is a CRITICAL sni routing bypass vulnerability (CWE-754: Improper Check for Unusual or Exceptional Conditions) affecting Netty libraries in the Maven ecosystem. The issue involves sni routing bypass via fragmented tls clienthello causing fallback to default sslcontext. Remediation is available by upgrading affected packages to version 4.2.17.Final, 4.1.137.Final.

CVE-2026-75595 at a Glance

CVE Identifier CVE-2026-75595

GHSA Identifier GHSA-c4c3-7fpv-j4q5

Severity CRITICAL

CVSS v3.1 Base Score N/A

CWE Classification CWE-754: Improper Check for Unusual or Exceptional Conditions

Affected Product Netty

Package Ecosystem Maven

Published Date 2026-09-08

Last Updated 2026-09-08

How to Fix CVE-2026-75595

To remediate this vulnerability, upgrade all affected Netty dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

io.netty:netty-handler fixed: 4.2.17.Final

<dependency>
    <groupId>io.netty</groupId>
    <artifactId>netty-handler</artifactId>
    <version>4.2.17.Final</version>
</dependency>

io.netty:netty-handler fixed: 4.1.137.Final

<dependency>
    <groupId>io.netty</groupId>
    <artifactId>netty-handler</artifactId>
    <version>4.1.137.Final</version>
</dependency>

Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release:

mvn dependency:tree -Dincludes=io.netty

How to Check if You Are Affected

To determine whether your Java application is affected by CVE-2026-75595, inspect your build dependencies using the Maven Dependency Plugin:

mvn dependency:tree -Dincludes=io.netty

Analyze the terminal output to locate any version of Netty in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 4.2.17.Final), your application is affected.

Note: The mvn dependency:tree -Dincludes=io.netty command inspects the resolved dependency graph declared in your project's POM files; it does not perform automated security scanning or inspect runtime dynamic classpaths. Always verify runtime container classpaths and shaded JARs.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-754: Improper Check for Unusual or Exceptional Conditions, where the product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product. In Netty, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Summary 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: ```java 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.

CVE-2026-75595 Affected Netty Versions

2 component(s)

Fixed Versions:

4.2.17.Final4.1.137.Final

Product / Package	Ecosystem	Affected Versions	Fixed Version
io.netty:netty-handler maven/io.netty/netty-handler	Maven	≥ 4.2.0.Final < 4.2.17.Final	4.2.17.Final
io.netty:netty-handler maven/io.netty/netty-handler	Maven	< 4.1.137.Final	4.1.137.Final

References

GitHub Advisory / Wiki ADVISORY

https://github.com/netty/netty/security/advisories/GHSA-c4c3-7fpv-j4q5

NVD — CVE-2026-75595 NVD

https://nvd.nist.gov/vuln/detail/CVE-2026-75595

Netty Source Repository REPOSITORY

https://github.com/netty/netty/pull/17213

Netty Source Repository REPOSITORY

https://github.com/netty/netty/pull/17217

Security Fix Commit FIX COMMIT

https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7

Security Fix Commit FIX COMMIT

https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961

Netty Source Repository REPOSITORY

https://github.com/netty/netty

Netty Release RELEASE

https://github.com/netty/netty/releases/tag/netty-4.1.137.Final

Netty Release RELEASE

https://github.com/netty/netty/releases/tag/netty-4.2.17.Final

Frequently Asked Questions

What is CVE-2026-75595?

CVE-2026-75595 is a security vulnerability in Netty involving SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext under CWE-754: Improper Check for Unusual or Exceptional Conditions.

Which Netty versions are affected?

Affected versions include releases prior to 4.2.17.Final, 4.1.137.Final across affected packages: io.netty:netty-handler.

What version fixes CVE-2026-75595?

Upgrading to 4.2.17.Final, 4.1.137.Final (or the latest patched release for your respective branch) resolves the vulnerability.

Is netty-handler affected?

Yes, io.netty:netty-handler is affected in releases prior to 4.2.17.Final, 4.1.137.Final. Upgrade your dependency to 4.2.17.Final, 4.1.137.Final or later.

What CWE is associated with CVE-2026-75595?

CVE-2026-75595 is classified under CWE-754: Improper Check for Unusual or Exceptional Conditions (The product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product.).

How can I check Maven dependencies for CVE-2026-75595?

Run the dependency tree command: 'mvn dependency:tree -Dincludes=io.netty'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.

Machine-readable OSV JSON OSV v1.5 Schema

Normalized schema output

View Raw JSON

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-c4c3-7fpv-j4q5",
  "modified" : "2026-09-08T18:16:14Z",
  "published" : "2026-09-08T18:16:14Z",
  "aliases" : [ "CVE-2026-75595" ],
  "summary" : "Netty: SNI Routing Bypass via Fragmented TLS ClientHello Causing Fallback to Default SslContext",
  "details" : "### Summary\nA 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.\n\n### Details\nIn `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:\n\n```java\nif (handshakeLength == -1) {\n    if (readerIndex + 4 > endOffset) {\n        // Need more data to read HandshakeType and handshakeLength (4 bytes)\n        return;\n    }\n```\n\nWhen 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.\n\n### Impact\nSNI routing bypass. Escalates to an unauthenticated mTLS bypass only when:\n- mTLS is enforced solely via per-SNI SslContext (clientAuth=REQUIRE)\n- the default/fallback SslContext is permissive (clientAuth=NONE/OPTIONAL)\n- no secondary peer-certificate verification exists at the application layer.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "io.netty:netty-handler"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "4.2.0.Final"
      }, {
        "fixed" : "4.2.17.Final"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.2.16.Final"
    }
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "io.netty:netty-handler"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.1.137.Final"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 4.1.136.Final"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/netty/netty/security/advisories/GHSA-c4c3-7fpv-j4q5"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-75595"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/netty/netty/pull/17213"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/netty/netty/pull/17217"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/netty/netty"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/netty/netty/releases/tag/netty-4.1.137.Final"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/netty/netty/releases/tag/netty-4.2.17.Final"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-475", "CWE-754" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-08T18:16:14Z",
    "nvd_published_at" : "2026-08-19T21:17:37Z"
  }
}

CVSS V4

N/A CRITICAL

Attack Vector N/A

Attack Complexity N/A

Privileges Required N/A

User Interaction N/A

Scope N/A

Confidentiality N/A

Integrity N/A

Availability N/A

Vector String

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N

Advisory Metadata

Source ID: GHSA-c4c3-7fpv-j4q5
Schema Version: 1.4.0
Published: 2026-09-08 18:16
Last Modified: 2026-09-08 18:16
Database Record File: advisories/github-reviewed/2026/09/GHSA-c4c3-7fpv-j4q5/GHSA-c4c3-7fpv-j4q5.json