GHSA-c98p-7wgm-6p64
Tornado: Quadratic DoS via Repeated Header Coalescing
At a glance
- Published
- Updated
- CVE
- CVE-2025-67725
- Aliases
- PYSEC-2025-266
- CVSS_V3
- CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Description
## Summary The `HTTPHeaders.add` method in Tornado accumulates values using string concatenation when the same header name is repeated. Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. Given Tornado's single event loop architecture, a single maliciously crafted HTTP request can block the server's event loop for an extended period, causing a Denial of Service (DoS). **Severity:** **High** if `max_header_size` has been increased from its default, **low** if it has its default value of 64KB.
Affected packages
| Package | Versions |
|---|---|
| PyPI · tornado | introduced 0, fixed 6.5.3 |
References
- https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64 WEB
- https://nvd.nist.gov/vuln/detail/CVE-2025-67725 ADVISORY
- https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd WEB
- https://github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2025-266.yaml WEB
- https://github.com/tornadoweb/tornado PACKAGE
- https://github.com/tornadoweb/tornado/releases/tag/v6.5.3 WEB
Sources
| Source | Licence |
|---|---|
| OSV.dev | CC-BY 4.0 — GitHub Advisory Database |