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CVE-2026-75596 high CVSS 7.5

Vulnerability in io.netty:netty-handler (CVE-2026-75596)

Summary

Vulnerability in io.netty:netty-handler (CVE-2026-75596). Risk of unauthorized operations or information disclosure. Exploitable via io.netty.handler.ssl.SniHandler. Mitigation: upgrade to 4.1.137.Final or later.

Original description

The description as published by the source.

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)
  • handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:174 (writeBytes re-copy)
  • codec-base/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java:294 (cumulation retention)

Exploit Path

1. TCP connection → SniHandler → SslClientHelloHandler.decode
2. First record: TLS handshake header declaring large ClientHello length (e.g., 4096 bytes)
3. Attacker sends thousands of tiny follow-on handshake records (1 byte each)
4. On each fragment: handshakeBuffer.clear() + writeBytes() re-copie…

Source: NIST National Vulnerability Database NVD GitHub Advisory

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-75596 has been found in io.netty:netty-handler.
Attackers can target a specific entry point like io.netty.handler.ssl.SniHandler over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 7.5/10.
What to do: upgrade io.netty:netty-handler to 4.1.137.Final or later.
If unsure, ask your IT team or search "io.netty:netty-handler CVE-2026-75596" on the vendor's site.

CVE-2026-75596 (io.netty:netty-handler) — CWE-407 / CVSS v3 7.5
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: io.netty.handler.ssl.SniHandler / SniHandler
Patched: 4.1.137.Final — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-407) exists in io.netty:netty-handler. Attackers reach the vulnerable code path via io.netty.handler.ssl.SniHandler without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of io.netty:netty-handler. Attack surface: io.netty.handler.ssl.SniHandler / SniHandler.

🔥 Severity

Severity: High (CVSS 7.5/10). Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 4.1.137.Final.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Audit SBOM/dependencies to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    Audit SBOM/dependencies for affected components.

    資産台帳・SBOM・依存マニフェストで io.netty:netty-handler を使っているサービスとバージョンを洗い出す。

  2. 2
    Apply patch patch 
    Upgrade io.netty:netty-handler to 4.1.137.Final

    ステージング環境で 4.1.137.Final に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

maven io.netty:netty-handler
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"4.1.137.Final"}]}]

References

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