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Medium, CVSS 6.9amended

Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing

CVE-2026-75596GHSA-fccg-mwvh-qqg4

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (io.netty:netty-handler 4.2.17.Final, 4.1.137.Final), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • Maven io.netty:netty-handler

    Affected
    >= 4.2.0.Final, <= 4.2.16.Final<= 4.1.136.Final
    Fixed in
    4.2.17.Final, 4.1.137.Final

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
6.9 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N · score source
EPSS
  • CVE-2026-75596: 0.37% estimated exploitation activity in the next 30 days; percentile 31.40 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary Netty's default SNI entrypoint reparses and recopies previously received ClientHello fragments on every additional TLS handshake record. A remote peer can send a small first record that advertises a large ClientHello length and then drip the body in many tiny records, causing superlinear (quadratic) CPU work before the handshake completes. With 4095 one-byte fragments, the handler recopies 8,386,560 bytes from only 24,579 bytes on the wire — a 341× amplification ratio. Affected Entrypoints - io.netty.handler.ssl.SniHandler — default constructors - io.netty.handler.ssl.SslClientHelloHandler — pre-handshake ClientHello aggregation path Vulnerable Code Locations - handler/src/main/java/io/netty/handler/ssl/SniHandler.java:85 - handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:75 (decode entry) - handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:165 (handshakeBuffer.clear) -…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-fccg-mwvh-qqg4

  2. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 1 · created ·

    Published from osv record GHSA-fccg-mwvh-qqg4

Agent brief

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# Patch Next advisory brief: CVE-2026-75596

Advisory: Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing
IDs: CVE-2026-75596, GHSA-fccg-mwvh-qqg4
Record: https://patchnext.com/advisories/cve-2026-75596 (revision 3, source checked 26 Oct 2026, 16:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: io.netty:netty-handler 4.2.17.Final, 4.1.137.Final. A vendor fix does not mean a deployment is fixed.

## Affected components
- Maven io.netty:netty-handler: affected >= 4.2.0.Final, <= 4.2.16.Final; <= 4.1.136.Final; fixed in 4.2.17.Final, 4.1.137.Final

## Suggested inspection (read-only)
- Maven io.netty:netty-handler: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on io.netty:netty-handler.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-fccg-mwvh-qqg4 (OSV record GHSA-fccg-mwvh-qqg4 lists affected ranges: Maven io.netty:netty-handler >=4.2.0.Final =0 <4.1.137.Final (fixed))
- Verified: https://github.com/advisories/GHSA-fccg-mwvh-qqg4 (GitHub-reviewed advisory GHSA-fccg-mwvh-qqg4 lists affected ranges: Maven io.netty:netty-handler >=4.2.0.Final =0 <=4.1.136.Final)

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