The compilation daemon in Scala before 2.10.7, 2.11.x before 2.11.12, and 2.12.x before 2.12.4 uses weak permissions for private files in /tmp/scala-devel/${USER:shared}/scalac-compile-server-port, which allows local users to write to arbitrary class files and consequently gain privileges.
https://security.gentoo.org/glsa/201812-08
https://github.com/scala/scala/pull/6128
https://github.com/scala/scala/pull/6120