Tornado 6.5.2 DoS via HTTPHeaders.add O(n2) Header Concatenation
CVE-2025-67725 Published on December 12, 2025

Tornado is Vulnerable to Quadratic DoS via Repeated Header Coalescing
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.

NVD

Vulnerability Analysis

CVE-2025-67725 can be exploited with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality and integrity, and a high impact on availability.

Attack Vector:
NETWORK
Attack Complexity:
LOW
Privileges Required:
NONE
User Interaction:
NONE
Scope:
UNCHANGED
Confidentiality Impact:
NONE
Integrity Impact:
NONE
Availability Impact:
HIGH

Weakness Type

What is a Resource Exhaustion Vulnerability?

The software does not properly control the allocation and maintenance of a limited resource, thereby enabling an actor to influence the amount of resources consumed, eventually leading to the exhaustion of available resources.

CVE-2025-67725 has been classified to as a Resource Exhaustion vulnerability or weakness.


Products Associated with CVE-2025-67725

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Canonical Ubuntu Linux
 

Affected Versions

tornadoweb tornado Version < 6.5.3 is affected by CVE-2025-67725

Exploit Probability

EPSS
0.31%
Percentile
54.07%

EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.