Moodle Authenticated Spelling Binary RCE

Moodle allows an authenticated user to define spellcheck settings via the web interface.
The user can update the spellcheck mechanism to point to a system-installed aspell binary.
By updating the path for the spellchecker to an arbitrary command, an attacker can run
arbitrary commands in the context of the web application upon spellchecking requests.

This module also allows an attacker to leverage another privilege escalation vuln.
Using the referenced XSS vuln, an unprivileged authenticated user can steal an admin sesskey
and use this to escalate privileges to that of an admin, allowing the module to pop a shell
as a previously unprivileged authenticated user.

This module was tested against Moodle version 2.5.2 and 2.2.3.