Add desktop_linux_privilege_escalation module

This module steals the user password of an administrative user on a desktop Linux system when it is entered for unlocking the screen or for doing administrative actions using policykit. Then it escalates to root privileges using sudo and the stolen user password.

It exploits the design weakness that there is no trusted channell for transferring the password from the keyboard to the actual password verificatition against the shadow file (which is running as root since /etc/shadow is only readable to the root user). Both screensavers (xscreensaver/gnome-screensaver) and policykit use a component running under the current user account to query for the password and then pass it to a setuid-root binary to do the password verification. Therefore it is possible to inject a password stealer after compromising the user account. Since sudo requires only the user password (and not the root password of the system), stealing the user password of an administrative user directly allows escalating to root privileges.

Please note that you have to start a handler as a background job before running this exploit since the exploit will only create a shell when the user actually enters the password (which may be hours after launching the exploit). Using exploit/multi/handler with the option ExitOnSession set to false should do the job.