CVE-2026-116577: Tornado: Quadratic CPU usage when parsing HTTP headers with line folding

Vulnerability summary

CVE-2026-116577 is a medium (CVSS 5.3) Inefficient Algorithmic Complexity vulnerability in tornadoweb tornado. EPSS estimates a 0.38% chance of exploitation in the next 30 days, higher than 32.4% of scored vulnerabilities.

Timeline

1. 26-10-2026
   PublishedVulnerability first disclosed.
2. 31-10-2026
   Last modifiedVulnerability information updated.

Description

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.

• CWE-407Inefficient Algorithmic ComplexityAn algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.
• CWE-770Allocation of Resources Without Limits or ThrottlingThe product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.