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SLAC Computer Security - X Windows
Revised 28 Feb 2002
The X Window System poses a serious security risk if it is not properly secured. An X11 "display" is the X11 server running on your desktop, and includes the screen, keyboard and mouse. If your X11 display is insecure, it will allow a program running anywhere on the Internet to connect to it and the connection may be completely invisible to you. Once connected, that program has full access to your display, which means that it can:
- View and copy your screen's contents, using standard X11 utility programs;
- Monitor your keystrokes;
- Remotely control any Netscape browser on your desktop; and
- Forge keystrokes as if you were typing them yourself (although not all X11 applications are susceptible to this).
The best defense is to prevent unwanted connections in the first place. Thus the rule:
- NEVER USE "xhost +" -- It completely disables your display's security.
NOTE: SLAC blocks direct Internet access to on-site X11 displays.
This means that an X11 application that runs on an off-site host can not point its display back to your desktop at SLAC -- at least not directly. However, there are two ways that you can run offsite X11 applications in a reasonably secure manner:
- Use ssh (secure shell), a program that runs on SLAC Unix, NT, or Mac systems, to tunnel an X11 connection through the firewall. The offsite host must have the sshd daemon installed. Or
- Use mxconns, a "proxy" program that runs on SLAC Unix systems, to relay the X11 connection from the offsite application to your desktop.
- Follow these links for more information about ssh and mxconns.
For more information about the security weaknesses inherent in the X11 Window system and how to improve X11 security, see the following documents:
Owner: SLAC Computer Security |