Ruby insecure file permissions in the CGI session management

According to a Debian Security Advisory:

Andres Salomon noticed a problem in the CGI session
management of Ruby, an object-oriented scripting language.
CGI::Session’s FileStore (and presumably PStore […])
implementations store session information insecurely.
They simply create files, ignoring permission issues.
This can lead an attacker who has also shell access to the
webserver to take over a session.