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.
JetBrains Ktor framework (created using the Kotlin IDE template) versions before 1.1.0 were resolving artifacts using an http connection during the build process, potentially allowing an MITM attack. This issue was fixed in Kotlin plugin version 1.3.30.