GHSA-3hv7-mjh2-fv65

## Summary

`HTTPServerRequest.__init__` in `tornado/httputil.py` parses the URL query string via `parse_qs_bytes()` with no field-count limit — while the sibling POST-body parsing path (`parse_body_arguments`) received a `max_num_fields=1000` cap added earlier in this exact same release (v6.5.8, commit `8d6363ed`), explicitly to bound parsing cost for the identical underlying primitive. This leaves the query-string path with the resource-exhaustion exposure the body-path fix was meant to close.

**File**: `tornado/httputil.py`, line 553 (`HTTPServerRequest.__init__`)

### Root Cause

```python # tornado/httputil.py:553 (before fix) self.arguments = parse_qs_bytes(self.query, keep_blank_values=True) ```

Compare with the POST-body path fixed one commit earlier in the same release:

```python # tornado/httputil.py:1038-1041 uri_arguments = parse_qs_bytes( body, keep_blank_values=True, max_num_fields=config.urlencoded.max_arguments, # default 1000 ) ```

Both call sites funnel through the same `tornado.escape.parse_qs_bytes` (a thin wrapper over `urllib.parse.parse_qs`), which is exactly why `max_num_fields` was added to `urllib.parse.parse_qsl` upstream — to let frameworks bound field count. The fix was applied only to the body path; the query-string path was missed.

The request line + headers together are capped at `max_header_size` (default 65536 bytes), so this is not literally unbounded, but a single ~64KB request line can carry thousands of short `key=value` pairs — far beyond the 1000-field limit the maintainer judged appropriate for the structurally identical body case.

### Attack Scenario

1. Attacker sends a `GET` request whose query string is packed with thousands of short fields (e.g. `k0=1&k1=1&...&k7799=1`, ~61KB), fitting comfortably under `max_header_size`. No authentication, cookies, or prior state required. 2. Tornado accepts and parses this with no field-count cap, unlike the equivalent POST-body request (which is correctly rejected with 400 once >1000 fields are present). 3. Parsing thousands of fields is CPU work performed synchronously inside Tornado's single-threaded `IOLoop`. Several such requests in flight concurrently stall the event loop, delaying processing of *all* other connections on that loop — not just the attacker's own request.

### Verification (dynamic, local reproduction against v6.5.8)

Ran the unmodified v6.5.8 source directly (no external dependencies needed) with a minimal `tornado.web.Application` on `127.0.0.1:8888`.

- Identical 7800-field/~61KB payload sent as GET query string → `200 OK`; sent as POST body (`application/x-www-form-urlencoded`) → `400 Bad Request` (correctly rejected by the existing `max_num_fields` body-path limit). This confirms the asymmetry directly. - Per-request parse cost: baseline (`/?a=1`) averaged 1.86ms; the 7800-field query string averaged 25.1ms (~13x). - Event-loop-blocking amplification (raw-socket test, isolating server-side stall from client overhead): with 10 sequential baseline probe requests fired with no load, average latency was 1.47ms (max 5.9ms). With 5 concurrent 61KB/7800-field requests in flight, the same baseline probes averaged 13.0ms (max 118.1ms) — an **8.9x average slowdown** for unrelated clients, produced by ~305KB of unauthenticated attacker traffic.

### Impact

All Tornado servers/applications are affected — this triggers on every request with a query string, independent of application/handler logic. An unauthenticated, unprivileged remote attacker can measurably degrade response times for all other clients sharing the same `IOLoop`, using a small amount of bandwidth and no special conditions. This is an availability/DoS concern; no confidentiality or integrity impact.

### Recommended Fix

```python # tornado/httputil.py — HTTPServerRequest.__init__ if uri is not None: self.path, sep, self.query = uri.partition("?") try: self.arguments = parse_qs_bytes( self.query, keep_blank_values=True, max_num_fields=_DEFAULT_PARSE_BODY_CONFIG.urlencoded.max_arguments, ) except ValueError as e: raise HTTPInputError("Invalid query string: %s" % e) from e ```

This reuses the existing `ParseUrlEncodedConfig.max_arguments` default (1000) via the module's `_DEFAULT_PARSE_BODY_CONFIG`, matching the POST-body limit and honoring any global override via `set_parse_body_config()`. The `try/except` is necessary because — unlike `parse_body_arguments`, which already wraps its call and converts `ValueError` into a clean `HTTPInputError`/400 — the query-string call site currently has no such handling, so without it, a request exceeding the limit would raise an uncaught `ValueError` instead of a clean 400.

Verified: with the fix applied, requests with ≤1000 query-string fields are unaffected; requests with >1000 fields are rejected with `400 Bad Request` (consistent with the POST-body behavior); Tornado's own `httputil_test` and `web_test` suites (256 tests) pass unchanged.