The X server architecture, designed long time ago by some happy hippies who just thought all the apps are good and non-malicious, simply allows any GUI application to control any other one. No bugs, no exploits, no tricks, are required. This is all by design. One application can sniff or inject keystrokes to another one, can take snapshots of the screen occupied by windows belonging to another one, etc.
Is Wayland more secure than X11? If so, where can I find information about Wayland's supposed GUI isolation mechanism?