GMS-2023-1908: Tornado vulnerable to HTTP request smuggling via improper parsing of `Content-Length` fields and chunk lengths

Summary

Tornado interprets -, +, and _ in chunk length and Content-Length values, which are not allowed by the HTTP RFCs. This can result in request smuggling when Tornado is deployed behind certain proxies that interpret those non-standard characters differently. This is known to apply to older versions of haproxy, although the current release is not affected.

Details

Tornado uses the int constructor to parse the values of Content-Length headers and chunk lengths in the following locations:

<code>tornado/http1connection.py:445</code>

      self._expected_content_remaining = int(headers["Content-Length"])

<code>tornado/http1connection.py:621</code>

        content_length = int(headers["Content-Length"]) # type: Optional[int]

<code>tornado/http1connection.py:671</code>

      chunk_len = int(chunk_len_str.strip(), 16)

Because int("0_0") == int("+0") == int("-0") == int("0"), using the int constructor to parse and validate strings that should contain only ASCII digits is not a good strategy.