{"id":"CVE-2026-82397","summary":"Tornado: Urlencoded body parsing omits max_num_fields, so one request can stall the event loop","details":"Tornado 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.","aliases":["GHSA-mpf4-983q-p7j4"],"modified":"2026-09-02T03:47:29.949450175Z","published":"2026-08-31T21:28:13.958Z","database_specific":{"cna_assigner":"GitHub_M","cwe_ids":["CWE-1284","CWE-400"],"osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/82xxx/CVE-2026-82397.json"},"references":[{"type":"WEB","url":"https://github.com/tornadoweb/tornado/releases/tag/v6.5.8"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/82xxx/CVE-2026-82397.json"},{"type":"ADVISORY","url":"https://github.com/tornadoweb/tornado/security/advisories/GHSA-mpf4-983q-p7j4"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-82397"},{"type":"FIX","url":"https://github.com/tornadoweb/tornado/commit/8d6363ed7b69d5f0da806efe34d256627a2191de"},{"type":"FIX","url":"https://github.com/tornadoweb/tornado/pull/3704"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://github.com/tornadoweb/tornado","events":[{"introduced":"0"},{"fixed":"8d6363ed7b69d5f0da806efe34d256627a2191de"},{"fixed":"a55abe3e3bf59994f29b2f7084c46341f0d4f6a7"}],"database_specific":{"source":["AFFECTED_FIELD","REFERENCES"],"extracted_events":[{"introduced":"0"},{"fixed":"6.5.8"}]}}],"versions":["v6.5.7","v6.5.6","v6.5.5","v6.5.4","v6.5.3","v6.5.2","v6.5.1","v6.5.0","v6.5.0b1","v6.4.1","v6.4.0","v6.4.0b1","v6.3.1","v6.3.0","v6.3.0b1","v6.2.0","v6.2.0b2","v6.2.0b1","v6.1.0","v6.1.0b2","v6.1.0b1","v6.0.0","v6.0.0b1","v5.1.0","v5.0.0","v4.5.0","v4.5.1","v4.4.1","v4.4.0","v4.4.0b1","v4.3.0","v4.3.0b2","v4.3.0b1","v4.2.0","v4.2.0b1","v4.1.0","v4.1.0b2","v4.1.0b1","v4.0.0","v4.0.0b3","v4.0.0b2","v4.0.0b1","v3.2.0","v3.2.0b2","v3.2.0b1","v3.1.0","v3.0.1","v3.0.0","v2.1.1","v2.3.0","v2.2.0","v2.1.0","v2.0.0","v1.2.0","v1.1.0","v1.0.0"],"database_specific":{"source":"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-82397.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"}]}