In JetBrains Kotlin before 1.4.21, a vulnerable Java API was used for temporary file and folder creation. An attacker was able to read data from such files and list directories due to insecure permissions.
In JetBrains Kotlin from 1.4-M1 to 1.4-RC (as Kotlin 1.3.7x is not affected by the issue. Fixed version is 1.4.0) there is a script-cache privilege escalation vulnerability due to kotlin-main-kts cached scripts in the system temp directory, which is shared by all users by default.