T 4.12 Lack of authentication possibilities between X Server and X Client

A particular requirement concerning the X-Windows system is that it should be used only in a safe environment since it offers all the users involved the opportunity to corrupt both the X client and the X server. The X server process is responsible for the input and output in a computer, and is not interested in the identity of the owner of the X client process with whom it is communicating. Thus, all X clients can access all data input on an X server, and the X server has no means by which to ascertain from which X client it receives data. Thus, for instance, the meltdown program simulates optical "melting" of the screen of any X server, and it is just as possible to read data of an xterm client or to transmit personal data to that client, i.e. make screen copies from another computer that runs on X-Windows.