X Server Detection

Overview

X Server detection refers to identifying the presence of an X Window System server, commonly known as X11 or simply X. X is a network-transparent window system which runs on Unix-like operating systems. Detecting an X server can reveal information about the graphical environment and potential security risks associated with it.

• Severity: Low to Moderate

Impact

• Information Disclosure: Detection can provide attackers with information about the graphical environment and running applications.
• Unauthorized Access: Misconfigured X servers can allow unauthorized users to capture keystrokes or take screenshots.
• Privilege Escalation: Exploiting vulnerabilities in the X server could potentially lead to elevated privileges on the system.

Cause

• Open Ports: X servers typically listen on TCP port 6000 by default, which can be detected through network scanning.
• Misconfiguration: Insecure or default configurations may expose the X server to unauthorized access.
• Legacy Systems: Older systems running outdated versions of X server may have known vulnerabilities.

Solution

Securing X Server

1. Disable TCP Listening:

   • Add the -nolisten tcp option to your X server startup script to prevent it from listening on a network port.

     startx -- -nolisten tcp

2. Restrict Access Control:

   • Use the xhost command to restrict access to the X server. For example, to allow only the local machine:

     xhost +local:

3. Update X Server:

   • Ensure you are using the latest version of the X server software, which includes security patches and improvements.

4. Use SSH Tunneling:

   • For remote X11 sessions, use SSH with X11 forwarding instead of allowing direct network connections to the X server.

     ssh -X user@remote_host

Monitoring and Detection

1. Network Scanning:

   • Regularly scan your network for open X server ports (6000+) using tools like nmap.

     nmap -p 6000-6005 <target>

2. Log Monitoring:

   • Monitor logs for unauthorized access attempts or unusual activity related to the X server.

Examples

Disabling TCP Listening

To disable TCP listening on an X server:

1. Edit the X server startup configuration, typically found in /etc/X11/xinit/xserverrc.

2. Add the -nolisten tcp option:

   exec /usr/bin/X -nolisten tcp "$@"

References

• X.org Security Advisories

Additional Resources

• Securing access to the X Window System