Tornado is Vulnerable to Quadratic DoS via Repeated Header Coalescing

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2025-67725
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2025-12-12 06:15:41 UTC
Updated2026-10-07 20:10:01 UTC
DescriptionTornado 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.

Risk And Classification

Primary CVSS: v3.1 7.5 HIGH from [email protected]

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

EPSS: 0.005590000 probability, percentile 0.447660000 (date 2026-10-07)

Problem Types: CWE-400 | CWE-400 CWE-400: Uncontrolled Resource Consumption

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary7.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
3.1CNADECLARED7.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

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

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Tornadoweb Tornado All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Tornadoweb Tornado affected < 6.5.3 Not specified

References

ReferenceSourceLinkTags
github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6... [email protected] github.com Patch
github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64 [email protected] github.com Vendor Advisory
github.com/tornadoweb/tornado/releases/tag/v6.5.3 [email protected] github.com Release Notes
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

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