CVE-2025-67725
Publication date 12 December 2025
Last updated 8 January 2026
Ubuntu priority
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.
Status
| Package | Ubuntu Release | Status |
|---|---|---|
| python-tornado | 25.10 questing |
Fixed 6.4.2-3ubuntu0.2
|
| 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
|
Get expanded security coverage with Ubuntu Pro
Reduce your average CVE exposure time from 98 days to 1 day with expanded CVE patching, ten-years security maintenance and optional support for the full stack of open-source applications. Free for personal use.
Get Ubuntu Pro 30-day free trialNotes
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