How to remediate – X Font Service Detection

1. Introduction

The X Font Service Detection vulnerability indicates an X font service is running on a remote host. This service serves font files to clients using the X Window System. While not directly exploitable in many modern setups, it presents a potential attack surface as server-supplied fonts are now deprecated. Affected systems typically include those running graphical environments based on X11, such as older Linux distributions or specific applications relying on XFS. A successful exploit could lead to remote code execution, though this is unlikely without further vulnerabilities. Confidentiality, integrity and availability may be impacted if the service is compromised.

2. Technical Explanation

The vulnerability arises from an active X Font Service (xfs) daemon listening for connections. This daemon provides font files to requesting clients. The primary risk stems from the outdated nature of this protocol and its potential for exploitation, though modern systems often have mitigations in place. An attacker could potentially exploit vulnerabilities within the xfs daemon itself or use it as a pivot point for further attacks on the system.

• Root cause: The X Font Service is enabled and listening on the network, despite server-supplied fonts being deprecated.
• Exploit mechanism: An attacker could attempt to send malicious font files designed to exploit vulnerabilities in the xfs daemon’s parsing or handling of font data. This might involve buffer overflows or other memory corruption issues.
• Scope: Systems running X11 with an active xfs daemon are affected, particularly older Linux distributions and applications specifically configured to use server-supplied fonts.

3. Detection and Assessment

Confirming the presence of a listening xfs daemon is the primary assessment step. This can be done quickly using network tools or by checking running processes.

• Quick checks: Use `netstat -tulnp | grep 6000` to check if anything is listening on port 6000, the default XFS port.
• Scanning: Nessus plugin ID 139785 can detect this issue. This is an example only and may require updating.
• Logs and evidence: Check system logs for messages related to xfs or font server activity. Specific log files vary by distribution, but `/var/log/syslog` or similar are good starting points.

netstat -tulnp | grep 6000

4. Solution / Remediation Steps

The recommended solution is to disable the X Font Service if it’s not actively required. Limiting incoming traffic can also reduce risk, but disabling the service provides a more complete fix.

4.1 Preparation

• Roll back plan: Re-enable the service if applications are affected and no alternative font configuration is available. A change window may be needed for critical systems.

4.2 Implementation

1. Step 1: Stop the xfs daemon using `systemctl stop xfs`.
2. Step 2: Disable the service to prevent it from starting automatically on boot with `systemctl disable xfs`.
3. Step 3: Verify the service is stopped and disabled with `systemctl status xfs`.

4.3 Config or Code Example

Before

systemctl status xfs
● xfs.service - X Font Server
   Loaded: loaded (/lib/systemd/system/xfs.service; enabled; vendor preset: disabled)
   Active: active (running) since ...

After

systemctl status xfs
● xfs.service - X Font Server
   Loaded: loaded (/lib/systemd/system/xfs.service; disabled)
   Inactive: inactive (dead) since ...

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help mitigate risks associated with unnecessary services and outdated protocols.

• Least privilege: Run services with the minimum necessary privileges to reduce impact if exploited.
• Safe defaults: Disable unused or deprecated services by default.

4.5 Automation (Optional)

#!/bin/bash
# Stop and disable X Font Service on multiple hosts via SSH
for host in $(cat /path/to/hostlist); do
  ssh $host "sudo systemctl stop xfs && sudo systemctl disable xfs"
done

5. Verification / Validation

• Post-fix check: Run `netstat -tulnp | grep 6000`. The output should be empty.
• Re-test: Re-run the initial detection command (`netstat -tulnp | grep 6000`) to confirm no listening xfs daemon is present.
• Smoke test: Launch any applications that use fonts to ensure they display correctly.
• Monitoring: Monitor system logs for any errors related to font rendering or XFS activity, as an example alert.

netstat -tulnp | grep 6000

6. Preventive Measures and Monitoring

Regular security baselines and patch management are essential for preventing similar issues.

• Baselines: Update security baselines to include disabling unnecessary services like XFS.
• Pipelines: Incorporate checks in CI/CD pipelines to identify running deprecated services during deployment.
• Asset and patch process: Implement a regular review cycle for installed software and configurations.

7. Risks, Side Effects, and Roll Back

• Roll back: Re-enable the service using `systemctl enable xfs` and `systemctl start xfs`.

8. References and Resources

• Vendor advisory or bulletin: No specific vendor advisory is available for this general detection.
• NVD or CVE entry: No specific CVE exists for the mere presence of a listening XFS daemon.
• Product or platform documentation relevant to the fix: https://www.x.org/docs/FSProtocol/fsproto.pdf