GHSA-mpf4-983q-p7j4

## Summary

Tornado 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.

## Where it is

`tornado/escape.py`, at HEAD `e530031405e2154654dedc4c84d5656b557ea310`:

```python result = urllib.parse.parse_qs( qs, keep_blank_values, strict_parsing, encoding="latin1", errors="strict" ) ```

`max_num_fields` is the parameter CPython added for exactly this, and it is absent.

The 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`.

The 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.

## Impact

Denial 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.

## Suggested fix

Pass a bound:

```python result = urllib.parse.parse_qs( qs, keep_blank_values, strict_parsing, encoding="latin1", errors="strict", max_num_fields=max_num_fields, ) ```

with 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.

Lowering the default body cap for urlencoded specifically would help too, since 100 MB of form fields is not a shape any real client sends.

## Why I do not think this is a duplicate

The 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.