(the example given is key logging, not sure if there's other ways) You can get mouse events besides keystroke events, grab windows including the root window (s

(the example given is key logging, not sure if there's other ways) You can get mouse events besides keystroke events, grab windows including the root window (screenshots), you can send keystrokes to other applications. tl;dr assume no isolation between any X11 applications. Typing in your credentials in some terminal? You should trust all other X11 applications that are currently running, including the browser that runs untrusted programs all the time. (There is a workaround: start applications in separate Xephyr sessions.)