Waitress through version 1.3.1 allows request smuggling by sending the
Content-Length header twice. Waitress would header fold a double
Content-Length header and due to being unable to cast the now comma
separated value to an integer would set the Content-Length to 0 internally.
If two Content-Length headers are sent in a single request, Waitress would
treat the request as having no body, thereby treating the body of the
request as a new request in HTTP pipelining. This issue is fixed in
Waitress 1.4.0.