Tornado: Quadratic CPU usage when parsing HTTP headers with line folding

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-116577
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-26 21:18:14 UTC
Updated2026-10-26 21:18:14 UTC
DescriptionTornado 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.

Risk And Classification

Primary CVSS: v3.1 5.3 MEDIUM from [email protected]

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Problem Types: CWE-407 | CWE-770 | CWE-407 CWE-407: Inefficient Algorithmic Complexity | CWE-770 CWE-770: Allocation of Resources Without Limits or Throttling

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary5.3MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
3.1CNADECLARED5.3MEDIUMCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
Low

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Tornadoweb Tornado affected < 6.6 Not specified

References

ReferenceSourceLinkTags
github.com/tornadoweb/tornado/commit/1b7a1dc9f340f52de929852e8125aa12f25... [email protected] github.com
github.com/tornadoweb/tornado/pull/3765 [email protected] github.com
github.com/tornadoweb/tornado/releases/tag/v6.6.0 [email protected] github.com
github.com/tornadoweb/tornado/security/advisories/GHSA-pcg2-g6cj-q92c [email protected] github.com
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