The Camera Upload functionality in Plex Media Server through 1.18.2.2029 allows remote authenticated users to write files anywhere the user account running the Plex Media Server has permissions. This allows remote code execution via a variety of methods, such as (on a default Ubuntu installation) creating a .ssh folder in the plex user's home directory via directory traversal, uploading an SSH authorized_keys file there, and logging into the host as the Plex user via SSH.