What are some of the security concerns and reasons either for or against allowing X11 Forwarding. I have generally taken an approach of not allowing it under the blanket guise of security. Recently, I had a user indicated that they thought that the security implications of resulting from allowing X11 Forwarded sessions were negligible. I was curious to learn more about what the harm is in allowing X11 Forwarding and why one might want to allow it.
The implication of X11 forwarding is that it opens a channel from the server back to the client. In a simple SSH session, the client is more trusted than the server: anyone in control of the client can run commands on the server (assuming shell access), but the converse is not true. With X11 forwarding, the server is likely to gain shell access to the client.
In a text session, there is a limited channel from the server back to the client: the server determines the output that is displayed on the client, and can in particular try to exploit escape sequences in the terminal running on the client,
In an X11 session, the server can send X11 commands back to the client. X11 was not designed with security in mind, it was designed with the idea that all programs that you're displaying are run by you and hence trusted anyway. By default, SSH subjects commands from the server to restrictions through the X11 SECURITY extension. The SECURITY extension disables some obvious attacks such as keyboard grabs and key injection, but allows others like focus stealing.
Let's say I open a SSH connection to