Currently, in GMod, there exists a way to use HTML panels to read files anywhere in the system.

This Addon, when applied to a server or run by clients manually, will mitigate all known ways to abuse this exploit (PRs Welcome!). I expect there are more methods, but hopefully Rubat will push a fix quickly and this fix will become unnecessary.

This addon does not have any known side effects and should be safe to put on any server without disrupting legitimate panels.