CVE-2025-67726

Publication date 12 December 2025

Last updated 8 January 2026

Ubuntu priority

Cvss 3 Severity Score

Description

Tornado is a Python web framework and asynchronous networking library. Versions 6.5.2 and below use an inefficient algorithm when parsing parameters for HTTP header values, potentially causing a DoS. The _parseparam function in httputil.py is used to parse specific HTTP header values, such as those in multipart/form-data and repeatedly calls string.count() within a nested loop while processing quoted semicolons. If an attacker sends a request with a large number of maliciously crafted parameters in a Content-Disposition header, the server's CPU usage increases quadratically (O(n²)) during parsing. Due to Tornado's single event loop architecture, a single malicious request can cause the entire server to become unresponsive for an extended period. This issue is fixed in version 6.5.3.

Status

Show unmaintained releases

Package	Ubuntu Release	Status
python-tornado	25.10 questing	Fixed 6.4.2-3ubuntu0.2
	25.04 plucky	Fixed 6.4.2-1ubuntu0.25.04.3
	24.04 LTS noble	Fixed 6.4.0-1ubuntu0.4
	22.04 LTS jammy	Fixed 6.1.0-3ubuntu0.1~esm4
	20.04 LTS focal	Fixed 6.0.3+really5.1.1-3ubuntu0.1~esm3
	18.04 LTS bionic	Fixed 4.5.3-1ubuntu0.2+esm2
	16.04 LTS xenial	Fixed 4.2.1-1ubuntu3.1+esm2

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Severity score breakdown

Parameter	Value
Base score	7.5 · High
Attack vector	Network
Attack complexity	Low
Privileges required	None
User interaction	None
Scope	Unchanged
Confidentiality	None
Integrity impact	None
Availability impact	High
Vector	CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References

Related Ubuntu Security Notices (USN)

USN-7950-1
Tornado vulnerabilities
8 January 2026