Tornado: Urlencoded body parsing omits max_num_fields, so one request can stall the event loop

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-82397
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-31 22:17:22 UTC
Updated2026-08-31 22:17:22 UTC
DescriptionTornado is a Python web framework and asynchronous networking library. Prior to 6.5.8, Tornado parses application/x-www-form-urlencoded request bodies with urllib.parse.parse_qs in tornado/escape.py without passing max_num_fields. RequestHandler._execute in tornado/web.py parses the body before handler dispatch through HTTPServerRequest._parse_body and parse_body_arguments in tornado/httputil.py, so an unauthenticated request body containing millions of separator-delimited fields can synchronously stall the single-threaded event loop and delay every connection. The body is bounded only by max_buffer_size, which defaults to 104857600 bytes. This issue is fixed in version 6.5.8.

Risk And Classification

Primary CVSS: v3.1 7.5 HIGH from [email protected]

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

Problem Types: CWE-400 | CWE-1284 | CWE-400 CWE-400: Uncontrolled Resource Consumption | CWE-1284 CWE-1284: Improper Validation of Specified Quantity in Input

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary7.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
3.1CNADECLARED7.5HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS v3.1 Breakdown

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

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

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Tornadoweb Tornado affected < 6.5.8 Not specified

References

ReferenceSourceLinkTags
github.com/tornadoweb/tornado/pull/3704 [email protected] github.com
github.com/tornadoweb/tornado/commit/8d6363ed7b69d5f0da806efe34d256627a2... [email protected] github.com
github.com/tornadoweb/tornado/security/advisories/GHSA-mpf4-983q-p7j4 [email protected] github.com
github.com/tornadoweb/tornado/releases/tag/v6.5.8 [email protected] github.com
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