IT Baseline Protection Manual - The Information Warfare Site

Threats Catalogue Deliberate Acts Remarks
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T 4.12 Lack of authentication possibilities between X
Server and X Client
Without suitable security mechanisms, such as, for example, "magic cookies"
or use of Secure Shell, the X Windows system especially should only be used
in a trusted environment. Without security enforcing functions it is possible
for any participating user to corrupt both the X client and the X server. The X
server process, which is responsible for the input and output on a computer,
has no means of detecting the identity of the owner of the X client process
which is communicating with it. In this way all X clients can access all data
input on an X server, and the X server has no means of telling from which X
client it is receiving data. Thus, for example, the meltdown program simulates
optical "melting" of the screen of any X server, while it equally possible to
read data of an xterm client or to send own data to that client, i.e. make screen
copies from another computer that runs on X Windows.
Examples
- With the xspy tool it is possible to automatically record keyboard inputs
remotely on an xterm client.
- Windows which are displayed by an aggressor on an X server are visually
no different from those of the intended X client. In this way an aggressor
could implant false information or provoke the input of sensitive
information with the aid of "imposter" windows.
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IT-Baseline Protection Manual: Oktober 2000