In JetBrains Rider versions 2019.3 EAP2 through 2019.3 EAP7, there were unsigned binaries provided by the Windows installer. This issue was fixed in release version 2019.3.
In Ktor before 1.3.0, request smuggling is possible when running behind a proxy that doesn't handle Content-Length and Transfer-Encoding properly or doesn't handle \n as a headers separator.