How to remediate – X Server Unauthenticated Access: Screenshot

1. Introduction

The X Server Unauthenticated Access: Screenshot vulnerability means a remote attacker can connect to an X server and capture screenshots of the connected host’s display. This compromises confidentiality as sensitive information shown on screen could be viewed without permission. Systems running an X server with TCP connections enabled are usually affected, especially those providing remote access or graphical applications over a network. Impact is high: potential loss of confidential data.

2. Technical Explanation

The vulnerability occurs because the X server accepts incoming TCP connections without proper authentication. An attacker can establish a connection and request a screenshot of the display. The main precondition for exploitation is that port 6000 (or another configured X server port) must be open and accessible from the network. This is tracked as CVE-1999-0526.

• Root cause: Lack of authentication on incoming TCP connections to the X server.
• Exploit mechanism: An attacker connects to the X server via TCP, sends commands to request a screenshot, and receives the image data. For example, using xwd or similar tools.
• Scope: Linux systems running an X server are affected. Older versions of X11 are particularly vulnerable.

3. Detection and Assessment

You can confirm a system is vulnerable by checking if the X server accepts TCP connections. A thorough method involves attempting to connect remotely and request a screenshot.

• Quick checks: Use netstat -tulnp | grep :6000 (or the configured port) to see if the X server process is listening on TCP.
• Scanning: Nessus plugin ID 21895 can detect this vulnerability as an example.
• Logs and evidence: Check system logs for connections to port 6000 or other X server ports, especially from unexpected sources.

netstat -tulnp | grep :6000

4. Solution / Remediation Steps

Restrict access to the X server port using the ‘xhost’ command or disable TCP connections entirely if the X client/server facility is not required.

4.1 Preparation

• Dependencies: Ensure you have access to the command line and appropriate permissions. Roll back by restoring the snapshot or re-enabling TCP connections.
• Change window: Consider scheduling this change during a maintenance window. Approval may be needed from system owners.

4.2 Implementation

1. Step 1: Restrict access to trusted hosts using ‘xhost +local:’. This allows only local connections.
2. Step 2: If remote access is required, add specific IP addresses or hostnames with ‘xhost +IP_ADDRESS’ or ‘xhost +HOSTNAME’.
3. Step 3: If the X client/server facility is not used, disable TCP connections by stopping and disabling the X server service.

4.3 Config or Code Example

Before

xhost +

After

xhost +local:

4.4 Security Practices Relevant to This Vulnerability

Least privilege is key here, limiting access only to trusted hosts. Input validation could prevent malicious commands being sent over the connection.

• Practice 1: Least privilege – restrict network access and user permissions to reduce potential impact if exploited.
• Practice 2: Network segmentation – isolate X servers from untrusted networks.

4.5 Automation (Optional)

#!/bin/bash
# Restrict X server access to localhost only
xhost +local:
echo "X server access restricted to localhost."

5. Verification / Validation

Confirm the fix by checking that remote connections are no longer accepted. Attempt a connection from an untrusted host.

• Post-fix check: Run xhost and verify only ‘local:’ is listed, or specific trusted hosts are present.
• Re-test: Repeat the initial detection steps (netstat) to confirm no remote connections are allowed.
• Smoke test: Ensure any local applications using the X server still function correctly.
• Monitoring: Monitor system logs for failed connection attempts to port 6000 from unexpected sources as an example.

xhost

6. Preventive Measures and Monitoring

Update security baselines to include restrictions on X server access. Implement regular patch cycles to address known vulnerabilities.

• Baselines: Update your system hardening baseline or CIS control to enforce restricted X server access.
• Pipelines: Consider using SAST tools to identify insecure configurations in infrastructure code.
• Asset and patch process: Review and apply security patches regularly, especially for critical services like the X server.

7. Risks, Side Effects, and Roll Back

Restricting access may break existing applications that require remote X server connections. Disabling the X server will prevent all graphical applications from running.

• Risk or side effect 2: Loss of functionality if the X server is disabled. Mitigation: Ensure alternative solutions are available if graphical applications are required.
• Roll back: Restore the system snapshot, re-enable the X server service, and revert ‘xhost’ settings to their original configuration.

8. References and Resources

• Vendor advisory or bulletin: No specific vendor advisory available for CVE-1999-0526.
• NVD or CVE entry: CVE-1999-0526
• Product or platform documentation relevant to the fix: CERT Advisory 704969