- Form-encoded ``POST`` bodies are now subject to a limit of 1000 arguments by default. This
  prevents a CPU and memory denial of service attack. This limit can be overridden via the
  `.set_parse_body_config` function. Thanks to `Arpit Jain <https://github.com/arpitjain099>`_ 
  for reporting this issue.
- Multipart parsing now rejects requests with an excessive number of parts earlier in the parsing
  process, limiting memory consumption. Thanks to `afldl <https://github.com/afldl>`_ for
  reporting this issue.
- The deprecated mixed-case arguments to `.RequestHandler.set_cookie` now enforce the same
  restrictions on invalid characters that were introduced in Tornado 6.5.5 for the standard
  lowercase arguments. Thanks to `sec-reex <https://github.com/sec-reex>`_ and
  `Arpit Jain <https://github.com/arpitjain099>`_ for reporting this issue.

Deprecations
~~~~~~~~~~~~

- The `.OpenIdMixin` class is deprecated and will be removed in Tornado 6.7. OpenID 2.0 is no
  longer widely supported by identity providers.