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.
Exploitability
AV:NAC:HPR:LUI:NScope
S:CImpact
C:HI:LA:N7.1/CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:L/A:NOther