Now You Know

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Now You Know…

This chapter has laid out the basics of the X Window System architecture and has hopefully given you an idea of some of the potential security risks you take when running X-based applications. The power to run graphical applications remotely comes at a price.

Several X-related utilities can exploit some of these security risks. We mentioned xkey, which lets you monitor the keystrokes on an X server to which you have access (either legitimately or from a lack of authorization and access control). Another program, xwatchwin, will let you view the actual contents of the X server’s window, again assuming you have access. You can use a program called xscan to search networks for X servers that would be vulnerable to these kinds of attacks. All of these utilities are available for download at http://www.packetstormsecurity.com/.

Have we scared you away from running X yet? We hope not. You just need to remember four basic points when you’re running an X server to keep it as secure as possible:

• Avoid xhost access control if possible. It’s the least secure option you have. Use xauth variations instead.

• Run all your remote X applications back to your X server through an SSH tunnel.

• Turn off XDMCP unless you’re positive your network is private and trusted.

• If you’re running a local X server only and don’t need any remote capabilities, disable X’s ability to listen for incoming TCP connections on port 6000 and above. This can be done by passing the –nolisten tcp option to the actual X server’s command line, either in an xinitrc file or directly from startx using startx -- -nolisten tcp.

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