Vulnerability-Lookup

GHSA-C98P-7WGM-6P64

Vulnerability from github – Published: 2026-07-20 18:57 – Updated: 2026-07-20 18:57

Summary

Tornado: Quadratic DoS via Repeated Header Coalescing

Details

Summary

The HTTPHeaders.add method in Tornado accumulates values using string concatenation when the same header name is repeated. Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity.

Given Tornado's single event loop architecture, a single maliciously crafted HTTP request can block the server's event loop for an extended period, causing a Denial of Service (DoS).

Severity: High if max_header_size has been increased from its default, low if it has its default value of 64KB.

Severity

7.5 (High)


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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "tornado"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "6.5.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2025-67725"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-400"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-20T18:57:34Z",
    "nvd_published_at": "2025-12-12T06:15:41Z",
    "severity": "HIGH"
  },
  "details": "## Summary\n\nThe `HTTPHeaders.add` method in Tornado accumulates values using string concatenation when the same header name is repeated. Due to Python string immutability, each concatenation copies the entire string, resulting in O(n\u00b2) time complexity.\n\nGiven Tornado\u0027s single event loop architecture, a single maliciously crafted HTTP request can block the server\u0027s event loop for an extended period, causing a Denial of Service (DoS).\n\n**Severity:**  **High** if `max_header_size` has been increased from its default, **low** if it has its default value of 64KB.",
  "id": "GHSA-c98p-7wgm-6p64",
  "modified": "2026-07-20T18:57:34Z",
  "published": "2026-07-20T18:57:34Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-67725"
    },
    {
      "type": "WEB",
      "url": "https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2025-266.yaml"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/tornadoweb/tornado"
    },
    {
      "type": "WEB",
      "url": "https://github.com/tornadoweb/tornado/releases/tag/v6.5.3"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Tornado: Quadratic DoS via Repeated Header Coalescing"
}

CVE-2025-67725 (GCVE-0-2025-67725)

Vulnerability from cvelistv5 – Published: 2025-12-12 05:49 – Updated: 2025-12-18 18:51

Title

Tornado is Vulnerable to Quadratic DoS via Repeated Header Coalescing

Summary

Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, a single maliciously crafted HTTP request can block the server's event loop for an extended period, caused by the HTTPHeaders.add method. The function accumulates values using string concatenation when the same header name is repeated, causing a Denial of Service (DoS). Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. The severity can vary from high if max_header_size has been increased from its default, to low if it has its default value of 64KB. This issue is fixed in version 6.5.3.

Severity

7.5 (High)


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

SSVC

Exploitation: none Automatable: yes Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-400 - Uncontrolled Resource Consumption

Assigner

GitHub_M

References

3 references

URL	Tags
https://github.com/tornadoweb/tornado/security/ad…	x_refsource_CONFIRM
https://github.com/tornadoweb/tornado/commit/7714…	x_refsource_MISC
https://github.com/tornadoweb/tornado/releases/ta…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
tornadoweb	tornado	Affected: < 6.5.3

Show details on NVD website

JSON

To clipboard

{
  "containers": {
    "adp": [
      {
        "metrics": [
          {
            "other": {
              "content": {
                "id": "CVE-2025-67725",
                "options": [
                  {
                    "Exploitation": "none"
                  },
                  {
                    "Automatable": "yes"
                  },
                  {
                    "Technical Impact": "partial"
                  }
                ],
                "role": "CISA Coordinator",
                "timestamp": "2025-12-18T18:50:52.619221Z",
                "version": "2.0.3"
              },
              "type": "ssvc"
            }
          }
        ],
        "providerMetadata": {
          "dateUpdated": "2025-12-18T18:51:00.369Z",
          "orgId": "134c704f-9b21-4f2e-91b3-4a467353bcc0",
          "shortName": "CISA-ADP"
        },
        "title": "CISA ADP Vulnrichment"
      }
    ],
    "cna": {
      "affected": [
        {
          "product": "tornado",
          "vendor": "tornadoweb",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 6.5.3"
            }
          ]
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "value": "Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, a single maliciously crafted HTTP request can block the server\u0027s event loop for an extended period, caused by the HTTPHeaders.add method. The function accumulates values using string concatenation when the same header name is repeated, causing a Denial of Service (DoS).  Due to Python string immutability, each concatenation copies the entire string, resulting in O(n\u00b2) time complexity. The severity can vary from high if max_header_size has been increased from its default, to low if it has its default value of 64KB. This issue is fixed in version 6.5.3."
        }
      ],
      "metrics": [
        {
          "cvssV3_1": {
            "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"
          }
        }
      ],
      "problemTypes": [
        {
          "descriptions": [
            {
              "cweId": "CWE-400",
              "description": "CWE-400: Uncontrolled Resource Consumption",
              "lang": "en",
              "type": "CWE"
            }
          ]
        }
      ],
      "providerMetadata": {
        "dateUpdated": "2025-12-12T05:49:41.523Z",
        "orgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
        "shortName": "GitHub_M"
      },
      "references": [
        {
          "name": "https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64",
          "tags": [
            "x_refsource_CONFIRM"
          ],
          "url": "https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64"
        },
        {
          "name": "https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd"
        },
        {
          "name": "https://github.com/tornadoweb/tornado/releases/tag/v6.5.3",
          "tags": [
            "x_refsource_MISC"
          ],
          "url": "https://github.com/tornadoweb/tornado/releases/tag/v6.5.3"
        }
      ],
      "source": {
        "advisory": "GHSA-c98p-7wgm-6p64",
        "discovery": "UNKNOWN"
      },
      "title": "Tornado is Vulnerable to Quadratic DoS via Repeated Header Coalescing"
    }
  },
  "cveMetadata": {
    "assignerOrgId": "a0819718-46f1-4df5-94e2-005712e83aaa",
    "assignerShortName": "GitHub_M",
    "cveId": "CVE-2025-67725",
    "datePublished": "2025-12-12T05:49:41.523Z",
    "dateReserved": "2025-12-10T19:25:20.819Z",
    "dateUpdated": "2025-12-18T18:51:00.369Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2"
}

PYSEC-2025-266

Vulnerability from pysec - Published: 2025-12-12 06:15 - Updated: 2026-07-13 05:52

Details

Severity

7.5 (High)


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

Impacted products

Name	purl
tornado	pkg:pypi/tornado

Aliases

JSON

To clipboard

{
  "affected": [
    {
      "ecosystem_specific": {},
      "package": {
        "ecosystem": "PyPI",
        "name": "tornado",
        "purl": "pkg:pypi/tornado"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "6.5.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.2",
        "1.0",
        "1.1",
        "1.1.1",
        "1.2",
        "1.2.1",
        "2.0",
        "2.1",
        "2.1.1",
        "2.2",
        "2.2.1",
        "2.3",
        "2.4",
        "2.4.1",
        "3.0",
        "3.0.1",
        "3.0.2",
        "3.1",
        "3.1.1",
        "3.2",
        "3.2.1",
        "3.2.2",
        "4.0",
        "4.0.1",
        "4.0.2",
        "4.1",
        "4.1b2",
        "4.2",
        "4.2.1",
        "4.2b1",
        "4.3",
        "4.3b1",
        "4.3b2",
        "4.4",
        "4.4.1",
        "4.4.2",
        "4.4.3",
        "4.4b1",
        "4.5",
        "4.5.1",
        "4.5.2",
        "4.5.3",
        "4.5b1",
        "4.5b2",
        "5.0",
        "5.0.1",
        "5.0.2",
        "5.0a1",
        "5.0b1",
        "5.1",
        "5.1.1",
        "5.1b1",
        "6.0",
        "6.0.1",
        "6.0.2",
        "6.0.3",
        "6.0.4",
        "6.0a1",
        "6.0b1",
        "6.1",
        "6.1b1",
        "6.1b2",
        "6.2",
        "6.2b1",
        "6.2b2",
        "6.3",
        "6.3.1",
        "6.3.2",
        "6.3.3",
        "6.3b1",
        "6.4",
        "6.4.1",
        "6.4.2",
        "6.4b1",
        "6.5",
        "6.5.1",
        "6.5.2",
        "6.5b1"
      ]
    }
  ],
  "aliases": [
    "CVE-2025-67725",
    "GHSA-c98p-7wgm-6p64"
  ],
  "details": "Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, a single maliciously crafted HTTP request can block the server\u0027s event loop for an extended period, caused by the HTTPHeaders.add method. The function accumulates values using string concatenation when the same header name is repeated, causing a Denial of Service (DoS).  Due to Python string immutability, each concatenation copies the entire string, resulting in O(n\u00b2) time complexity. The severity can vary from high if max_header_size has been increased from its default, to low if it has its default value of 64KB. This issue is fixed in version 6.5.3.",
  "id": "PYSEC-2025-266",
  "modified": "2026-07-13T05:52:13.964136Z",
  "published": "2025-12-12T06:15:41.380Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://github.com/tornadoweb/tornado/releases/tag/v6.5.3"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64"
    },
    {
      "type": "FIX",
      "url": "https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd"
    }
  ],
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ]
}

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.

Detection rules are retrieved from Rulezet.

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.

Action not permitted

GHSA-C98P-7WGM-6P64

Summary

CVE-2025-67725 (GCVE-0-2025-67725)

PYSEC-2025-266

Tags

Sightings

Nomenclature