CVE-2026-82397: Tornado Urlencoded Body Parsing Omits Max_num_fields, Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-mpf4-983q-p7j4",
  "modified" : "2026-09-02T14:38:43Z",
  "published" : "2026-09-02T14:38:43Z",
  "aliases" : [ "CVE-2026-82397" ],
  "summary" : " Tornado: Urlencoded body parsing omits max_num_fields, so one request can stall the event loop",
  "details" : "## Summary\n\nTornado parses `application/x-www-form-urlencoded` bodies with `urllib.parse.parse_qs` and does not pass `max_num_fields`. A body made almost entirely of separators produces tens of millions of fields, and the parse happens on the event loop before the handler runs, so a single request stalls the whole server.\n\n## Where it is\n\n`tornado/escape.py`, at HEAD `e530031405e2154654dedc4c84d5656b557ea310`:\n\n```python\nresult = urllib.parse.parse_qs(\n    qs, keep_blank_values, strict_parsing, encoding=\"latin1\", errors=\"strict\"\n)\n```\n\n`max_num_fields` is the parameter CPython added for exactly this, and it is absent.\n\nThe path to it is entirely server-side and pre-dispatch. `RequestHandler._execute` parses the body at `tornado/web.py:1821`, which reaches `HTTPServerRequest._parse_body` at `tornado/httputil.py:636`, and the urlencoded branch of `parse_body_arguments` calls `parse_qs_bytes` at `tornado/httputil.py:1030`.\n\nThe size that reaches it is bounded only by the body cap, which defaults to the stream's `max_buffer_size` of 104857600 at `tornado/iostream.py:239`, applied as the request body default at `tornado/http1connection.py:136-140`. A 100 MB body of separators is around fifty million fields.\n\n## Impact\n\nDenial of service against the whole process, not one request. Tornado is single-threaded and the parse is synchronous on the event loop, so every other connection waits. No authentication is needed if any route accepts a form post, which is the normal case.\n\n## Suggested fix\n\nPass a bound:\n\n```python\nresult = urllib.parse.parse_qs(\n    qs, keep_blank_values, strict_parsing, encoding=\"latin1\", errors=\"strict\",\n    max_num_fields=max_num_fields,\n)\n```\n\nwith a conservative default and a way for applications to raise it. CPython raises `ValueError` when the limit is exceeded, which maps cleanly onto a 400.\n\nLowering the default body cap for urlencoded specifically would help too, since 100 MB of form fields is not a shape any real client sends.\n\n## Why I do not think this is a duplicate\n\nThe published tornado advisories cover out-of-bounds access in the C extension, unbounded accumulation of decompressed chunks in `AsyncHTTPClient`, the Authorization header surviving cross-origin redirects, credential leakage on curl handle reuse, and cookie attribute validation. The decompression one is the nearest in spirit and is on the client side; this is the server parsing a request body. The call is unchanged at HEAD.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "tornado"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "6.5.8"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 6.5.7"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "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" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/pull/3704"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/commit/8d6363ed7b69d5f0da806efe34d256627a2191de"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/tornadoweb/tornado"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/releases/tag/v6.5.8"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1284", "CWE-400" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-02T14:38:43Z",
    "nvd_published_at" : "2026-08-31T22:17:22Z"
  }
}