Executive brief
Tornado, a popular Python web framework and networking library, is vulnerable to a denial-of-service attack. An attacker can send a specially crafted web request containing an excessive number of form parts, causing the server to spend significant time processing the request. This can lead to the application becoming unresponsive to legitimate users, potentially causing service outages.
Technical details
In Tornado versions prior to 6.5.5, the multipart/form-data parser lacks a limit on the number of parts allowed in a request, constrained only by the overall 'max_body_size'. Because parsing occurs synchronously on the main event loop thread, an attacker can submit a request with a very large number of small parts to consume excessive CPU cycles. This results in a denial-of-service (DoS) condition by blocking the main thread. The vulnerability is addressed in version 6.5.5 by introducing a default limit of 100 parts per request and providing new configuration options via 'tornado.httputil.ParseMultipartConfig'.
Affected products
- TornadoWeb Tornado <= 6.5.4
Timeline
- 2026-03-10: disclosed: Vulnerability published to tornadoweb/tornado repository
- 2026-03-11: advisory: Published to NVD
- 2026-03-12: advisory: Published to GitHub Advisory Database
- 2026-03-12: patched: Version 6.5.5 released