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.

Metrics

No CVSS v4.0

Attack Vector Network

Attack Complexity Low

Privileges Required None

Scope Unchanged

Confidentiality Impact None

Integrity Impact None

Availability Impact High

User Interaction None

No CVSS v3.0

No CVSS v2

This CVE is not in the KEV list.

The EPSS score is 0.00433.

Key SSVC decision points have not yet been added.

Affected Vendors & Products

Vendors Products
Tornadoweb Subscribe
Tornado Subscribe

No data.

No data.

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Vendors Products
Tornadoweb Subscribe
Tornado Subscribe
Advisories

No advisories yet.

Fixes

Solution

No solution given by the vendor.

Workaround

No workaround given by the vendor.

References
Link Providers
https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd cve-icon cve-icon cve-icon
https://github.com/tornadoweb/tornado/releases/tag/v6.5.3 cve-icon cve-icon cve-icon
https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64 cve-icon cve-icon cve-icon
https://nvd.nist.gov/vuln/detail/CVE-2025-67725 cve-icon
https://www.cve.org/CVERecord?id=CVE-2025-67725 cve-icon
History

Sun, 14 Dec 2025 21:30:00 +0000

Type Values Removed Values Added
First Time appeared Tornadoweb
Tornadoweb tornado
Vendors & Products Tornadoweb
Tornadoweb tornado

Sat, 13 Dec 2025 00:15:00 +0000

Type Values Removed Values Added
References
Metrics threat_severity

None

 threat_severity

Moderate

Fri, 12 Dec 2025 06:00:00 +0000

Type Values Removed Values Added
Description 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.
Title Tornado is Vulnerable to Quadratic DoS via Repeated Header Coalescing
Weaknesses CWE-400
References
Metrics cvssV3_1

{'score': 7.5, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'}

Projects

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cve-icon MITRE

Status: PUBLISHED

Assigner: GitHub_M

Published:

Updated: 2025-12-12T05:49:41.523Z

Reserved: 2025-12-10T19:25:20.819Z

Link: CVE-2025-67725

cve-icon Vulnrichment

No data.

cve-icon NVD

Status : Undergoing Analysis

Published: 2025-12-12T06:15:41.380

Modified: 2025-12-12T15:17:31.973

Link: CVE-2025-67725

cve-icon Redhat

Severity : Moderate

Publid Date: 2025-12-12T05:49:41Z

Links: CVE-2025-67725 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2025-12-14T21:17:08Z

Weaknesses