CVE-2026-75596: Netty Fragmented ClientHello Records Trigger Quadratic Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-fccg-mwvh-qqg4",
  "modified" : "2026-09-08T18:14:56Z",
  "published" : "2026-09-08T18:14:56Z",
  "aliases" : [ "CVE-2026-75596" ],
  "summary" : "Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing",
  "details" : "### Summary\n\nNetty'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.\n\n### Affected Entrypoints\n\n- `io.netty.handler.ssl.SniHandler` — default constructors\n- `io.netty.handler.ssl.SslClientHelloHandler` — pre-handshake ClientHello aggregation path\n\n### Vulnerable Code Locations\n\n- `handler/src/main/java/io/netty/handler/ssl/SniHandler.java:85`\n- `handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:75` (decode entry)\n- `handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:165` (handshakeBuffer.clear)\n- `handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java:174` (writeBytes re-copy)\n- `codec-base/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java:294` (cumulation retention)\n\n### Exploit Path\n\n```\n1. TCP connection → SniHandler → SslClientHelloHandler.decode\n2. First record: TLS handshake header declaring large ClientHello length (e.g., 4096 bytes)\n3. Attacker sends thousands of tiny follow-on handshake records (1 byte each)\n4. On each fragment: handshakeBuffer.clear() + writeBytes() re-copies ALL accumulated body bytes\n5. Total bytes copied = n*(n+1)/2 where n = number of body bytes → quadratic\n6. Event-loop CPU exhausted before SslHandler takes over\n```\n\n### Impact\n\n- **Vulnerability Type:** Inefficient Algorithmic Complexity\n- An unauthenticated network attacker can drive disproportionate CPU consumption on the Netty event loop\n- Affects **all** Netty deployments using `SniHandler` for TLS termination (the default SNI path)\n- No privileges, user interaction, or special configuration required\n- Can degrade or stall TLS connection handling for all clients on the affected event loop\n- The attack requires only modest bandwidth (~25 KB) to trigger significant CPU work\n\n---\n### Credits\n\nFound by a security research team from the University of Sydney, focusing on detecting open source software vulnerabilities.\nLiyi Zhou: https://lzhou1110.github.io/\nZiyue Wang: https://zyy0530.github.io/\nStrick: https://str1ckl4nd.github.io/\nMaurice: https://maurice.busystar.org/\nChenchen Yu: https://7thparkk.github.io/",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "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-fccg-mwvh-qqg4"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-75596"
  }, {
    "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-407" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-08T18:14:56Z",
    "nvd_published_at" : "2026-08-19T21:17:37Z"
  }
}