The Hello World plugin for WordPress is vulnerable to Arbitrary File Reading in all versions up to, and including, 2.1.1 via the hello_world_lyric() function. This makes it possible for authenticated attackers, with subscriber-level access and above, to read the contents of arbitrary files on the server, which can contain sensitive information.
https://plugins.trac.wordpress.org/changeset/3161773/
https://plugins.trac.wordpress.org/browser/hello-world/tags/2.1.1/hello-world.php#L35
https://plugins.trac.wordpress.org/browser/hello-world/tags/2.1.1/hello-world.php#L113