medium5.3
CVE-2026-116577
Tornado: Quadratic CPU usage when parsing HTTP headers with line folding
Impact
Tornado is a Python web framework and asynchronous networking library. Prior to 6.6, Tornado's HTTP header parser accepts obsolete line folding and repeatedly concatenates short continuation lines, causing quadratic CPU consumption on the IOLoop thread. An unauthenticated client can submit a header block containing many obsolete line-folding continuations before application code runs, blocking other connections served by the process. The cost grows with max_header_size, and only availability is affected. This issue is fixed in version 6.6.
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| pip | tornado | < 6.6 | 6.6 |
Weakness classification
References
- NVD record
- CVE.org record
- GitHub advisory
- https://github.com/advisories/GHSA-pcg2-g6cj-q92c
- https://github.com/tornadoweb/tornado/security/advisories/GHSA-pcg2-g6cj-q92c
- https://nvd.nist.gov/vuln/detail/CVE-2026-116577
- https://github.com/tornadoweb/tornado/pull/3765
- https://github.com/tornadoweb/tornado/commit/1b7a1dc9f340f52de929852e8125aa12f25297c9
- https://github.com/tornadoweb/tornado/releases/tag/v6.6.0