CVE-2025-67725

7.5 HIGH 0.6% exploit odds Fix listed Denial of service Library

What it is

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.

At a glance

Exploitability: Can be exploited over the network without logging in. High risk of service disruption.
Exploit odds: 0.6% chance of exploitation in the next 30 days (EPSS).
Fix: A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

2025-12-12 06:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2026-10-07 20:10 UTC Last updatedMost recent change to the record

Why it ranks here

Exploited in the wild: no, CISA does not list it.
Exploit odds: 0.6% in the next 30 days (EPSS).
Severity: 7.5 HIGH (CVSS).
Fix: a patch or advisory is linked.

Details

Published: 2025-12-12 06:15 UTC
Last modified: 2026-10-07 20:10 UTC
Exploit probability: 0.6% · EPSS, next 30 days
Weakness: CWE-400 · Uncontrolled resource consumption
Record status: Analyzed

Affected

Tornadoweb Tornado < 6.5.3

Attack profile CVSS 3.1 · 7.5

Attack vector: Network
Complexity: Low
Privileges needed: None
User interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Raw vector

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

Is it in your code?

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Read https://cve-list.com/agent/cve/CVE-2025-67725.md and follow it. Check whether this project is affected by CVE-2025-67725. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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Fix and advisories

References

NVD record · CVE.org record