Vulnerability-Lookup

CVE-2026-75596 (GCVE-0-2026-75596)

Vulnerability from cvelistv5 – Published: 2026-08-19 20:56 – Updated: 2026-08-20 19:59

Title

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

Summary

Netty is an asynchronous, event-driven network application framework. Prior to 4.1.137.Final and 4.2.17.Final, the default io.netty.handler.ssl.SniHandler constructors use the pre-handshake ClientHello aggregation path in handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java at io.netty.handler.ssl.SslClientHelloHandler#decode, where handshakeBuffer.clear() and writeBytes() recopy all previously received body bytes for every additional TLS record. An unauthenticated remote peer can advertise a large ClientHello and deliver its body in thousands of tiny records, causing quadratic CPU work on the event loop before the TLS handshake completes and degrading TLS handling for other clients. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.

Severity

8.7 (High)


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

SSVC

Exploitation: none Automatable: yes Technical Impact: partial

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-08-20 19:59 UTC

CWE

CWE-407 - Inefficient Algorithmic Complexity

Assigner

GitHub_M CNA under the mitre root

CNA scorecard C 72/100 over 7994 records in the last 180 days details

References

7 references

URL	Tags
https://github.com/netty/netty/security/advisorie…	x_refsource_CONFIRM
https://github.com/netty/netty/pull/17213	x_refsource_MISC
https://github.com/netty/netty/pull/17217	x_refsource_MISC
https://github.com/netty/netty/commit/1b5abc6443b…	x_refsource_MISC
https://github.com/netty/netty/commit/9e051923910…	x_refsource_MISC
https://github.com/netty/netty/releases/tag/netty…	x_refsource_MISC
https://github.com/netty/netty/releases/tag/netty…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
netty	netty	Affected: < 4.1.137.Final Affected: >= 4.2.0.Final, < 4.2.17.Final	guessed

Show details on NVD website

JSON

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CLEANSTART-2026-OS42010 (CVE-2026-75596)

Vulnerability from cleanstart – Published: 2026-10-08 00:43 – Updated: 2026-10-09 04:22 – Source website

Summary

Netty is an asynchronous, event-driven network application framework

Details

Security vulnerability affects the apache-nifi-registry package. Netty is an asynchronous, event-driven network application framework.

Severity

7.5 (High)


                    
                      CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

8.7 (High)


                    
                      CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

References

URL

Type

	https://github.com/cleanstart-dev/cleanstart-secu…	ADVISORY
	https://osv.dev/vulnerability/CVE-2026-75596	WEB
	https://nvd.nist.gov/vuln/detail/CVE-2026-75596	WEB

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "CleanStart",
        "name": "apache-nifi-registry",
        "purl": "pkg:apk/cleanstart/apache-nifi-registry"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.9.0-r6"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "credits": [],
  "database_specific": {},
  "details": "Security vulnerability affects the apache-nifi-registry package. Netty is an asynchronous, event-driven network application framework.",
  "id": "CLEANSTART-2026-OS42010",
  "modified": "2026-10-09T04:22:25Z",
  "published": "2026-10-08T00:43:12.259272Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://github.com/cleanstart-dev/cleanstart-security-advisories/tree/main/advisories/2026/CLEANSTART-2026-OS42010.json"
    },
    {
      "type": "WEB",
      "url": "https://osv.dev/vulnerability/CVE-2026-75596"
    },
    {
      "type": "WEB",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-75596"
    }
  ],
  "related": [],
  "schema_version": "1.7.3",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Netty is an asynchronous, event-driven network application framework",
  "upstream": [
    "CVE-2026-75596"
  ]
}

CLEANSTART-2026-XL02389 (CVE-2026-75596)

Vulnerability from cleanstart – Published: 2026-09-29 00:37 – Updated: 2026-10-09 06:46 – Source website

Summary

Netty is an asynchronous, event-driven network application framework

Details

CVE-2026-75596 affects multiple packages. Netty is an asynchronous, event-driven network application framework. See references for individual vulnerability details.

Severity

7.5 (High)


                    
                      CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

8.7 (High)


                    
                      CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

References

URL

Type

	https://github.com/cleanstart-dev/cleanstart-secu…	ADVISORY
	https://osv.dev/vulnerability/CVE-2026-75596	WEB
	https://nvd.nist.gov/vuln/detail/CVE-2026-75596	WEB

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "CleanStart",
        "name": "elasticsearch",
        "purl": "pkg:apk/cleanstart/elasticsearch"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "9.2.8-r5"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "package": {
        "ecosystem": "CleanStart",
        "name": "elasticsearch",
        "purl": "pkg:apk/cleanstart/elasticsearch"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "9.4.5-r0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "credits": [],
  "database_specific": {},
  "details": "CVE-2026-75596 affects multiple packages. Netty is an asynchronous, event-driven network application framework. See references for individual vulnerability details.",
  "id": "CLEANSTART-2026-XL02389",
  "modified": "2026-10-09T06:46:06Z",
  "published": "2026-09-29T00:37:27.933661Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://github.com/cleanstart-dev/cleanstart-security-advisories/tree/main/advisories/2026/CLEANSTART-2026-XL02389.json"
    },
    {
      "type": "WEB",
      "url": "https://osv.dev/vulnerability/CVE-2026-75596"
    },
    {
      "type": "WEB",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-75596"
    }
  ],
  "related": [],
  "schema_version": "1.7.3",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Netty is an asynchronous, event-driven network application framework",
  "upstream": [
    "CVE-2026-75596"
  ]
}

FKIE_CVE-2026-75596

Vulnerability from fkie_nvd - Published: 2026-08-19 21:17 - Updated: 2026-10-26 10:10

Severity

7.5 (High) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Summary

References

URL	Tags
security-advisories@github.com	https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7	Patch
security-advisories@github.com	https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961	Patch
security-advisories@github.com	https://github.com/netty/netty/pull/17213	Issue Tracking, Patch
security-advisories@github.com	https://github.com/netty/netty/pull/17217	Issue Tracking, Patch
security-advisories@github.com	https://github.com/netty/netty/releases/tag/netty-4.1.137.Final	Release Notes
security-advisories@github.com	https://github.com/netty/netty/releases/tag/netty-4.2.17.Final	Release Notes
security-advisories@github.com	https://github.com/netty/netty/security/advisories/GHSA-fccg-mwvh-qqg4	Vendor Advisory

Impacted products

	Vendor	Product	Version
	netty	netty	*
	netty	netty	*

JSON

To clipboard

{
  "affected": [
    {
      "affectedData": [
        {
          "product": "netty",
          "vendor": "netty",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 4.1.137.Final"
            },
            {
              "status": "affected",
              "version": "\u003e= 4.2.0.Final, \u003c 4.2.17.Final"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
  "configurations": [
    {
      "nodes": [
        {
          "cpeMatch": [
            {
              "criteria": "cpe:2.3:a:netty:netty:*:*:*:*:*:*:*:*",
              "matchCriteriaId": "C2D95C1D-B022-435F-B4C7-33FC460064EA",
              "versionEndExcluding": "4.1.137",
              "vulnerable": true
            },
            {
              "criteria": "cpe:2.3:a:netty:netty:*:*:*:*:*:*:*:*",
              "matchCriteriaId": "80E3B839-CD5C-45D0-81C0-CABCE7A5D9AF",
              "versionEndExcluding": "4.2.17",
              "versionStartIncluding": "4.2.0",
              "vulnerable": true
            }
          ],
          "negate": false,
          "operator": "OR"
        }
      ]
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "Netty is an asynchronous, event-driven network application framework. Prior to 4.1.137.Final and 4.2.17.Final, the default io.netty.handler.ssl.SniHandler constructors use the pre-handshake ClientHello aggregation path in handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java at io.netty.handler.ssl.SslClientHelloHandler#decode, where handshakeBuffer.clear() and writeBytes() recopy all previously received body bytes for every additional TLS record. An unauthenticated remote peer can advertise a large ClientHello and deliver its body in thousands of tiny records, causing quadratic CPU work on the event loop before the TLS handshake completes and degrading TLS handling for other clients. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final."
    },
    {
      "lang": "es",
      "value": "Netty es un framework de aplicaci\u00f3n de red as\u00edncrono y basado en eventos. Antes de las versiones 4.1.137.Final y 4.2.17.Final, los constructores predeterminados de io.netty.handler.ssl.SniHandler utilizan la ruta de agregaci\u00f3n de ClientHello previa al handshake en handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java en io.netty.handler.ssl.SslClientHelloHandler#decode, donde handshakeBuffer.clear() y writeBytes() vuelven a copiar todos los bytes del cuerpo recibidos previamente por cada registro TLS adicional. Un par remoto no autenticado puede anunciar un ClientHello grande y entregar su cuerpo en miles de registros peque\u00f1os, causando un trabajo de CPU cuadr\u00e1tico en el bucle de eventos antes de que se complete el handshake TLS y degradando el manejo de TLS para otros clientes. Este problema se corrige en las versiones 4.1.137.Final y 4.2.17.Final."
    }
  ],
  "id": "CVE-2026-75596",
  "lastModified": "2026-10-26T10:10:00.140",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "HIGH",
          "baseScore": 7.5,
          "baseSeverity": "HIGH",
          "confidentialityImpact": "NONE",
          "integrityImpact": "NONE",
          "privilegesRequired": "NONE",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
          "version": "3.1"
        },
        "exploitabilityScore": 3.9,
        "impactScore": 3.6,
        "source": "nvd@nist.gov",
        "type": "Primary"
      }
    ],
    "cvssMetricV40": [
      {
        "cvssData": {
          "Automatable": "NOT_DEFINED",
          "Recovery": "NOT_DEFINED",
          "Safety": "NOT_DEFINED",
          "attackComplexity": "LOW",
          "attackRequirements": "NONE",
          "attackVector": "NETWORK",
          "availabilityRequirement": "NOT_DEFINED",
          "baseScore": 8.7,
          "baseSeverity": "HIGH",
          "confidentialityRequirement": "NOT_DEFINED",
          "exploitMaturity": "NOT_DEFINED",
          "integrityRequirement": "NOT_DEFINED",
          "modifiedAttackComplexity": "NOT_DEFINED",
          "modifiedAttackRequirements": "NOT_DEFINED",
          "modifiedAttackVector": "NOT_DEFINED",
          "modifiedPrivilegesRequired": "NOT_DEFINED",
          "modifiedSubAvailabilityImpact": "NOT_DEFINED",
          "modifiedSubConfidentialityImpact": "NOT_DEFINED",
          "modifiedSubIntegrityImpact": "NOT_DEFINED",
          "modifiedUserInteraction": "NOT_DEFINED",
          "modifiedVulnAvailabilityImpact": "NOT_DEFINED",
          "modifiedVulnConfidentialityImpact": "NOT_DEFINED",
          "modifiedVulnIntegrityImpact": "NOT_DEFINED",
          "privilegesRequired": "NONE",
          "providerUrgency": "NOT_DEFINED",
          "subAvailabilityImpact": "NONE",
          "subConfidentialityImpact": "NONE",
          "subIntegrityImpact": "NONE",
          "userInteraction": "NONE",
          "valueDensity": "NOT_DEFINED",
          "vectorString": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X",
          "version": "4.0",
          "vulnAvailabilityImpact": "HIGH",
          "vulnConfidentialityImpact": "NONE",
          "vulnIntegrityImpact": "NONE",
          "vulnerabilityResponseEffort": "NOT_DEFINED"
        },
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ],
    "ssvcV203": [
      {
        "source": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
        "ssvcData": {
          "id": "CVE-2026-75596",
          "options": [
            {
              "exploitation": "none"
            },
            {
              "automatable": "yes"
            },
            {
              "technicalImpact": "partial"
            }
          ],
          "role": "CISA Coordinator",
          "timestamp": "2026-08-20T19:59:07.317390Z",
          "version": "2.0.3"
        }
      }
    ]
  },
  "published": "2026-08-19T21:17:37.740",
  "references": [
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Patch"
      ],
      "url": "https://github.com/netty/netty/commit/1b5abc6443b63726c72cdd285af2feb7ddbb8ff7"
    },
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Patch"
      ],
      "url": "https://github.com/netty/netty/commit/9e0519239108a69b7e9bbc5e9182ee139a0d7961"
    },
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Issue Tracking",
        "Patch"
      ],
      "url": "https://github.com/netty/netty/pull/17213"
    },
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Issue Tracking",
        "Patch"
      ],
      "url": "https://github.com/netty/netty/pull/17217"
    },
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Release Notes"
      ],
      "url": "https://github.com/netty/netty/releases/tag/netty-4.1.137.Final"
    },
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Release Notes"
      ],
      "url": "https://github.com/netty/netty/releases/tag/netty-4.2.17.Final"
    },
    {
      "source": "security-advisories@github.com",
      "tags": [
        "Vendor Advisory"
      ],
      "url": "https://github.com/netty/netty/security/advisories/GHSA-fccg-mwvh-qqg4"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Analyzed",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-407"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Secondary"
    }
  ]
}

GHSA-FCCG-MWVH-QQG4

Vulnerability from github – Published: 2026-09-08 18:14 – Updated: 2026-09-08 18:14

Summary

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

Details

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-copies ALL accumulated body bytes
5. Total bytes copied = n*(n+1)/2 where n = number of body bytes → quadratic
6. Event-loop CPU exhausted before SslHandler takes over

Impact

Vulnerability Type: Inefficient Algorithmic Complexity
An unauthenticated network attacker can drive disproportionate CPU consumption on the Netty event loop
Affects all Netty deployments using SniHandler for TLS termination (the default SNI path)
No privileges, user interaction, or special configuration required
Can degrade or stall TLS connection handling for all clients on the affected event loop
The attack requires only modest bandwidth (~25 KB) to trigger significant CPU work

Credits

Found by a security research team from the University of Sydney, focusing on detecting open source software vulnerabilities. Liyi Zhou: https://lzhou1110.github.io/ Ziyue Wang: https://zyy0530.github.io/ Strick: https://str1ckl4nd.github.io/ Maurice: https://maurice.busystar.org/ Chenchen Yu: https://7thparkk.github.io/

Severity

6.9 (Medium)


                  
                    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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 4.2.16.Final"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "io.netty:netty-handler"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "4.2.0.Final"
            },
            {
              "fixed": "4.2.17.Final"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 4.1.136.Final"
      },
      "package": {
        "ecosystem": "Maven",
        "name": "io.netty:netty-handler"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "4.1.137.Final"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-75596"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-407"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-08T18:14:56Z",
    "nvd_published_at": "2026-08-19T21:17:37Z",
    "severity": "MODERATE"
  },
  "details": "### Summary\n\nNetty\u0027s 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 \u2014 a **341\u00d7 amplification** ratio.\n\n### Affected Entrypoints\n\n- `io.netty.handler.ssl.SniHandler` \u2014 default constructors\n- `io.netty.handler.ssl.SslClientHelloHandler` \u2014 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 \u2192 SniHandler \u2192 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 \u2192 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/",
  "id": "GHSA-fccg-mwvh-qqg4",
  "modified": "2026-09-08T18:14:56Z",
  "published": "2026-09-08T18:14:56Z",
  "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"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Netty: Fragmented ClientHello records trigger quadratic pre-handshake reassembly in default SNI parsing"
}

NCSC-2026-0444

Vulnerability from csaf_ncscnl - Published: 2026-10-21 14:43 - Updated: 2026-10-21 14:43

Summary

Kwetsbaarheden verholpen in Oracle Communications

Scope

31 vulnerabilities in this advisory, including CVE-2026-75596.

CVE-2026-75596

Netty versions prior to 4.1.137.Final and 4.2.17.Final contain a vulnerability in the SniHandler causing quadratic CPU usage during TLS handshakes by inefficient ClientHello fragment handling, exploited by sending many tiny TLS records, affecting Oracle Communications Unified Assurance 6.1.1 to 7.0.0 with critical impact.

9.8 (Critical)


                
                  CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Known affected: 4 products

Open the full advisory

OPENSUSE-SU-2026:11882-1

Vulnerability from csaf_opensuse - Published: 2026-09-24 00:00 - Updated: 2026-09-25 08:57

Summary

netty-4.1.138-1.1 on GA media

Severity

Moderate

Scope

30 vulnerabilities in this advisory, including CVE-2026-75596.

CVE-2026-75596

7.5 (High)


                
                  CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Recommended: 16 products

Open the full advisory

UBUNTU-CVE-2026-75596 (CVE-2026-75596)

Vulnerability from osv_ubuntu – Published: 2026-08-19 21:17 – Updated: 2026-10-17 09:52 – Source website

Details

Severity

8.7 (High)


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

7.5 (High)


                    
                      CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

N/A (UNKNOWN)


                    
                      medium

References

URL

Type

	https://ubuntu.com/security/CVE-2026-75596	REPORT
	https://www.cve.org/CVERecord?id=CVE-2026-75596	REPORT
	https://github.com/netty/netty/security/advisorie…	REPORT
	https://github.com/netty/netty/pull/17213	REPORT
	https://github.com/netty/netty/pull/17217	REPORT
	https://github.com/netty/netty/commit/1b5abc6443b…	REPORT
	https://github.com/netty/netty/commit/9e051923910…	REPORT
	https://github.com/netty/netty/releases/tag/netty…	REPORT
	https://github.com/netty/netty/releases/tag/netty…	REPORT

JSON

To clipboard

{
  "affected": [
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      "ecosystem_specific": {
        "binaries": [
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            "binary_name": "libnetty-java",
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        "name": "netty",
        "purl": "pkg:deb/ubuntu/netty@1:4.1.48-4+deb11u2ubuntu0.2+esm1?arch=source\u0026distro=esm-apps/jammy"
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        "1:4.1.48-16ubuntu0.1~esm2"
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  ],
  "aliases": [],
  "details": "Netty is an asynchronous, event-driven network application framework. Prior to 4.1.137.Final and 4.2.17.Final, the default io.netty.handler.ssl.SniHandler constructors use the pre-handshake ClientHello aggregation path in handler/src/main/java/io/netty/handler/ssl/SslClientHelloHandler.java at io.netty.handler.ssl.SslClientHelloHandler#decode, where handshakeBuffer.clear() and writeBytes() recopy all previously received body bytes for every additional TLS record. An unauthenticated remote peer can advertise a large ClientHello and deliver its body in thousands of tiny records, causing quadratic CPU work on the event loop before the TLS handshake completes and degrading TLS handling for other clients. This issue is fixed in versions 4.1.137.Final and 4.2.17.Final.",
  "id": "UBUNTU-CVE-2026-75596",
  "modified": "2026-10-17T09:52:42Z",
  "published": "2026-08-19T21:17:00Z",
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      "type": "REPORT",
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}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Vulnerability timeline

Sightings and recorded dates, grouped by day · UTC

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The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

CVE-2026-75596 (GCVE-0-2026-75596)

CLEANSTART-2026-OS42010 (CVE-2026-75596)

CLEANSTART-2026-XL02389 (CVE-2026-75596)

FKIE_CVE-2026-75596

GHSA-FCCG-MWVH-QQG4

Summary

Affected Entrypoints

Vulnerable Code Locations

Exploit Path

Impact

Credits

NCSC-2026-0444

OPENSUSE-SU-2026:11882-1

UBUNTU-CVE-2026-75596 (CVE-2026-75596)

Tags

Sightings

Nomenclature

Vulnerability timeline

Related by attack behaviour