Ruby is an Object Oriented, interpreted scripting language used for many system scripting tasks. It can also be used for CGI web applications.
The CGI::Session::FileStore implementation (and presumably CGI::Session::PStore), which allow data associated with a particular Session instance to be written to a file, writes to a file in /tmp with no regard for secure permissions. As a result, the file is left with whatever the default umask permissions are, which commonly would allow other local users to read the data from that session file.
Depending on the default umask, any data stored using these methods could be read by other users on the system.
By changing the default umask on the system to not permit read access to other users (e.g. 0700), one can prevent these files from being readable by other users.
All Ruby users should upgrade to the latest version:
# emerge sync
# emerge -pv ">=dev-lang/ruby-your_version"
# emerge ">=dev-lang/ruby-your_version"