How to remediate – X11 Server Unauthenticated Access

1. Introduction

The X11 Server Unauthenticated Access vulnerability means that a remote X11 server is accepting connections without checking who’s connecting. This allows an attacker on the network to potentially see what you’re doing, including keyboard presses and screenshots. Systems running an X11 server, typically Linux or Unix-like operating systems used for graphical applications, are affected. A successful exploit could compromise confidentiality, integrity, and availability of the remote host.

2. Technical Explanation

The root cause is a lack of access controls on the X11 server port (usually 6000). This allows any client to connect and interact with the display. An attacker can exploit this by connecting to the server and sending commands to capture keyboard input, mouse movements, or even display arbitrary content. CVE-1999-0526 details this issue.

• Root cause: Missing access control checks on incoming X11 connections.
• Exploit mechanism: An attacker connects to the X11 server and uses standard X11 protocol commands to interact with the display. For example, they could use ‘xprop’ to view window properties or capture screenshots using ‘import’.
• Scope: Linux and Unix-like systems running an X11 server are affected. Older versions of Xorg are particularly vulnerable.

3. Detection and Assessment

You can check if a system is vulnerable by seeing if connections are accepted on port 6000. A thorough method involves checking the ‘xhost’ output.

• Quick checks: Use `netstat -tulnp | grep :6000` to see if anything is listening on port 6000.
• Scanning: Nessus plugin ID 21894 can detect this vulnerability, but results should be verified manually.
• Logs and evidence: Check system logs for connections to the X11 server port (usually in /var/log/syslog or /var/log/auth.log). Look for suspicious connection attempts from unknown IP addresses.

netstat -tulnp | grep :6000

4. Solution / Remediation Steps

Restrict access to the X11 server port using the ‘xhost’ command or disable TCP if the facility is not used.

4.1 Preparation

• Ensure you have a way to revert the changes if needed. A rollback plan involves restoring the original ‘xhost’ settings or re-enabling TCP.
• Changes should be made during a scheduled maintenance window with appropriate approval from IT management.

4.2 Implementation

1. Step 1: Restrict access to trusted clients using `xhost +local:` to allow only local connections.
2. Step 2: If remote access is required, specify the IP addresses or hostnames of allowed clients with `xhost +` or `xhost +hostname`.
3. Step 3: To disable all X11 connections, use `xhost -` to remove all hosts from the access list.

4.3 Config or Code Example

Before

xhost +

After

xhost +local:

4.4 Security Practices Relevant to This Vulnerability

• Practice 1: Least privilege – only allow necessary connections to the X11 server, reducing the impact if exploited.
• Practice 2: Secure defaults – configure systems with restrictive access controls by default instead of permissive settings.

4.5 Automation (Optional)

#!/bin/bash
# Script to restrict X11 access to localhost only
xhost - # Remove all existing hosts
xhost +local: # Allow local connections
echo "X11 access restricted to localhost."

5. Verification / Validation

Confirm the fix by checking ‘xhost’ output and attempting a connection from a remote host.

• Post-fix check: Run `xhost` and verify that only trusted clients are listed or that no hosts are allowed. Expected output should show ‘+local:’ if restricted to local connections.
• Re-test: Re-run the `netstat -tulnp | grep :6000` command from section 3 and confirm it is still listening, but attempts to connect remotely fail.
• Smoke test: Verify that graphical applications continue to function correctly on the local host.
• Monitoring: Monitor system logs for failed X11 connection attempts from unauthorized IP addresses.

xhost

6. Preventive Measures and Monitoring

Update security baselines and implement patch management processes to prevent this issue.

• Baselines: Update your system security baseline to include a requirement for restrictive ‘xhost’ settings or disabling the X11 server if not needed.
• Pipelines: Include checks in CI/CD pipelines to ensure that new systems are configured with secure defaults, such as restricted X11 access.
• Asset and patch process: Implement a regular patch management cycle for all systems, including updates to the Xorg server.

7. Risks, Side Effects, and Roll Back

Restricting access may break remote graphical applications. Reverting involves restoring the original ‘xhost’ settings.

• Risk or side effect 1: Restricting access might prevent legitimate remote X11 connections. Mitigation is to add trusted IP addresses or hostnames using `xhost +`.
• Risk or side effect 2: Disabling the X11 server will break all graphical applications. Mitigation is to re-enable it if needed.
• Roll back: Step 1: Run `xhost +` to allow connections from any host. Step 2: If the X11 server was disabled, re-enable it using your system’s service management tools (e.g., `systemctl start display-manager`).

8. References and Resources

• Vendor advisory or bulletin: No specific vendor advisory available for this general vulnerability.
• NVD or CVE entry: CVE-1999-0526
• Product or platform documentation relevant to the fix: X.Org Security Page