Vulnerability · Published Dec 12, 2025 · Assigned by GitHub_M
CVE-2025-67725Tornado is Vulnerable to Quadratic DoS via Repeated Header Coalescing
High 7.5
In plain English
A high-severity uncontrolled resource consumption in tornado from tornadoweb. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 47% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| AlmaLinux:8 | pcs | 0.10.18-2.el8_10.8.alma.1 | ALSA-2026:0930 |
| AlmaLinux:8 | pcs-snmp | 0.10.18-2.el8_10.8.alma.1 | ALSA-2026:0930 |
| Alpine:v3.23 | py3-tornado | 6.5.4-r0 | ALPINE-CVE-2025-67725 |
| Alpine:v3.24 | py3-tornado | 6.5.4-r0 | ALPINE-CVE-2025-67725 |
| Debian:12 | python-tornado | 6.2.0-3+deb12u4 | DEBIAN-CVE-2025-67725 |
| Debian:13 | python-tornado | 6.4.2-3+deb13u2 | DEBIAN-CVE-2025-67725 |
| Debian:14 | python-tornado | 6.5.4-0.1 | DEBIAN-CVE-2025-67725 |
| PyPI | tornado | 6.5.3 | PYSEC-2025-266 |
| PyPI | tornado | 6.5.3 | GHSA-c98p-7wgm-6p64 |
| Rocky Linux:8 | pcs | 0:0.10.18-2.el8_10.8 | RLSA-2026:0930 |
| Ubuntu:24.04:LTS | python-tornado | 6.4.0-1ubuntu0.4 | UBUNTU-CVE-2025-67725 |
| Ubuntu:25.10 | python-tornado | 6.4.2-3ubuntu0.2 | UBUNTU-CVE-2025-67725 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| tornadoweb | tornado | < 6.5.3 |
Weakness: CWE-400 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H