How to remediate – Network Time Protocol (NTP) Mode 6 Scanner

1. Introduction

The Network Time Protocol (NTP) Mode 6 Scanner vulnerability means a server is responding to requests for information it shouldn’t be. This can allow attackers to use your server in denial of service attacks against others. Systems commonly affected are servers running NTP, particularly older versions or those with default configurations. Impact on confidentiality is unlikely, integrity is not directly affected, but availability could be severely impacted through reflected denial of service.

2. Technical Explanation

The vulnerability occurs because the NTP server allows unauthenticated queries using mode 6. Attackers can send crafted requests to these servers and spoof the source IP address to appear as the target of a distributed denial of service (DDoS) attack. An attacker needs network connectivity to the vulnerable NTP server. There is no specific CVE associated with this general issue, but https://ntpscan.shadowserver.org provides details on exposed servers.

• Root cause: The NTP server is configured to respond to mode 6 (monlist) queries without authentication or restriction.
• Exploit mechanism: An attacker sends a mode 6 query to the vulnerable server, spoofing the source IP address of the intended victim. The server responds with a large amount of data to the spoofed address, amplifying the attack traffic.
• Scope: Servers running NTP versions prior to recent updates are most affected. Default configurations often enable mode 6 responses.

3. Detection and Assessment

You can check if your system is vulnerable by confirming it responds to NTP mode 6 queries, or by using external scanning tools.

• Quick checks: Use the command `ntpq -p` on a Linux server. If any remote peers are listed, investigate further.
• Scanning: The ShadowServer NTP Scan can identify exposed servers. This is an external scan and should be used with caution.
• Logs and evidence: Check system logs for excessive network traffic related to port 123 (NTP). Look for unusual outbound responses from the NTP service.

ntpq -p

4. Solution / Remediation Steps

The best way to fix this is to restrict mode 6 queries on your NTP server.

4.1 Preparation

• A change window may be needed for critical systems. Approval from a senior IT administrator might be required.

4.2 Implementation

1. Step 1: Edit the NTP configuration file (usually `/etc/ntp.conf` or similar).
2. Step 2: Add the line `restrict default nomonlist`. This prevents external clients from querying mode 6.
3. Step 3: Restart the NTP service to apply the changes. For example, use the command `systemctl restart ntp`.

4.3 Config or Code Example

Before

# Default configuration (may allow monlist)
restrict default nomodify notrap nopeer noquery

After

# Restrict mode 6 queries
restrict default nomonlist nomodify notrap nopeer noquery

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue.

• Practice 1: Least privilege – restrict access to services only to those who need it, reducing the impact if exploited.
• Practice 2: Safe defaults – configure NTP with restrictive settings by default, preventing unnecessary exposure.

4.5 Automation (Optional)

If you use configuration management tools like Ansible, you can automate this change.

---
- name: Restrict NTP mode 6 queries
  lineinfile:
    path: /etc/ntp.conf
    regexp: '^restrict default '
    line: 'restrict default nomonlist nomodify notrap nopeer noquery'
  become: true
  notify: Restart NTP service
- name: Ensure NTP service is restarted
  service:
    name: ntp
    state: restarted
  become: true

5. Verification / Validation

Confirm the fix by checking that mode 6 queries are no longer answered.

• Post-fix check: Run `ntpq -p` again. No remote peers should be listed if the restriction is working correctly.
• Re-test: Re-run the ShadowServer NTP Scan to confirm your server is no longer identified as vulnerable.
• Monitoring: Monitor NTP service logs for any errors or unexpected behaviour. A simple log query looking for “denied” messages related to mode 6 could flag regressions.

ntpq -p

6. Preventive Measures and Monitoring

Update security baselines and consider adding checks in your CI/CD pipeline.

• Baselines: Update your server security baseline to include the `restrict default nomonlist` setting for NTP.
• Pipelines: Add a check to your configuration management system (e.g., Ansible, Chef) to ensure this setting is present on all NTP servers.
• Asset and patch process: Review NTP configurations regularly as part of your asset management or vulnerability patching cycle.

7. Risks, Side Effects, and Roll Back

Restricting mode 6 queries shouldn’t cause service disruption, but always test thoroughly.

• Risk or side effect 1: In rare cases, legitimate time synchronization clients might be affected if they rely on mode 6. Monitor logs for any issues.
• Risk or side effect 2: Incorrect configuration could prevent NTP from functioning at all. Ensure you have a roll back plan in place.
• Roll back: Restore the original NTP configuration file and restart the service. If necessary, revert to a snapshot of the server.

8. References and Resources

Links to official advisories and documentation.

• Vendor advisory or bulletin: Check your NTP vendor’s website for specific guidance on mode 6 restrictions.
• NVD or CVE entry: While there isn’t a single CVE, search the National Vulnerability Database for related NTP vulnerabilities.
• Product or platform documentation relevant to the fix: Refer to your operating system’s NTP configuration documentation (e.g., Red Hat, Debian).