GHSA-r4j5-9xpp-3985

### Summary When Tornado reads an HTTP/1.1 body that uses chunked transfer encoding, it processes all chunks that have already arrived without yielding to the event loop. A client can send a body made of millions of 1-byte chunks, which blocks the event loop (and therefore every other connection served by that process) for the entire time it takes to parse the body. A body near the default `max_body_size` of 100 MB can block the process for close to a minute, and the attacker can repeat or parallelize these requests. Bodies made of tiny chunks also consumed far more memory than their size: a handler that does not use `@stream_request_body` stored each chunk as a separate object, using roughly 20 times as much memory as the bytes received. ### Impact All Tornado HTTP servers are affected when they receive a request body for a handler that reads it, which includes any handler that does not reject the request before the body is read. No authentication is required. Lowering `max_body_size` reduces the maximum blocking time and memory use for each request but does not remove the issue. `SimpleAsyncHTTPClient` is affected in the same way when it reads a response from a malicious server. ### Patches Fixed in Tornado 6.6. Tornado now yields to the event loop periodically while reading any body, combines small chunks before passing them to `data_received`, and stores non-streaming bodies more compactly. Applications using `@stream_request_body` should note that the pieces passed to `data_received` no longer correspond to HTTP chunk boundaries. ### Workarounds - Run Tornado behind a reverse proxy that buffers request bodies and forwards them with a `Content-Length` header instead of chunked encoding. - Lower `max_body_size` (for the whole server or for individual handlers) to limit the impact of each request. ### Credit Reported by [uttej-badwane](https://github.com/uttej-badwane).