How to remediate – DNS over TLS Server Detection

1. Introduction

A DNS server is listening on the remote host over TLS. This means that Domain Name System (DNS) queries are being encrypted, which can help protect against eavesdropping but also introduces a potential attack surface if not managed correctly. Systems affected are typically those running DNS server software, such as BIND, PowerDNS or Microsoft DNS Server. A successful exploit could lead to information disclosure and potentially man-in-the-middle attacks impacting confidentiality, integrity, and availability of DNS resolution.

2. Technical Explanation

The vulnerability occurs when a DNS server is configured to accept TLS connections without appropriate restrictions or monitoring. An attacker can connect to the DNS server over TLS and potentially intercept or manipulate DNS traffic if the server’s certificate isn’t properly validated, or access is not restricted. There are no known CVEs associated with simply *running* DNS-over-TLS; rather, misconfigurations of the service itself pose a risk.

• Root cause: The DNS server is configured to listen on TLS without sufficient security measures in place.
• Exploit mechanism: An attacker connects to the DNS server over TLS and attempts to query for records or exploit vulnerabilities in the TLS configuration.
• Scope: Any system running a DNS server with TLS enabled, including BIND, PowerDNS, Microsoft DNS Server on Windows, and other DNS implementations.

3. Detection and Assessment

You can confirm if a system is vulnerable by checking for the presence of a listening TLS port on the standard DNS port (53). A thorough method involves examining the server’s configuration files.

• Quick checks: Use `netstat` or `ss` to check for TCP port 53 in a listening state. For example, on Linux: netstat -tulnp | grep ':53'
• Scanning: Nessus plugin ID 14869 can detect DNS over TLS servers. This is an example only.
• Logs and evidence: Check DNS server logs for TLS handshake events or errors related to certificate validation. Log file locations vary by DNS software (e.g., /var/log/syslog, Windows Event Logs).

netstat -tulnp | grep ':53'

4. Solution / Remediation Steps

The following steps provide a way to fix the issue by disabling or restricting access to the DNS service.

4.1 Preparation

• Ensure you have a rollback plan in place by keeping copies of the original configuration files. A roll back involves restoring the previous config file and restarting the service.
• A change window may be needed to minimize disruption. Approval from system owners might be required.

4.2 Implementation

1. Step 1: Disable DNS over TLS if it is not required. This usually involves modifying the DNS server’s configuration file (e.g., named.conf for BIND).
2. Step 2: If DNS over TLS must be enabled, restrict access to internal hosts only using firewall rules or access control lists (ACLs) on the DNS server.
3. Step 3: Restart the DNS service to apply the changes.

4.3 Config or Code Example

Before

options {
  listen-on port 53 { any; };
};

After

options {
  listen-on port 53 { localhost; };
};

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue.

• Practice 1: Least privilege – only allow necessary access to the DNS server, reducing the impact if exploited.
• Practice 2: Secure defaults – configure DNS servers with secure settings by default, minimizing the attack surface.

4.5 Automation (Optional)

# Example Ansible playbook snippet to restrict DNS access via firewall
- name: Restrict DNS access to internal network
  firewalld:
    zone: public
    rule: drop
    port: 53/tcp
    permanent: true
    state: enabled

5. Verification / Validation

Confirm the fix by checking that the DNS server is no longer listening on TLS externally.

• Post-fix check: Run netstat -tulnp | grep ':53' and verify that it only listens on localhost or internal network interfaces.
• Re-test: Re-run the initial `netstat` command to confirm that external access is blocked.
• Smoke test: Verify that DNS resolution still works for internal clients.
• Monitoring: Monitor DNS server logs for any unexpected TLS connection attempts from external sources.

netstat -tulnp | grep ':53'

6. Preventive Measures and Monitoring

Update security baselines to include secure DNS configurations.

• Baselines: Update your security baseline or policy to require restricted access for DNS servers with TLS enabled (for example, a CIS control).
• Pipelines: Add checks in CI/CD pipelines to ensure that DNS server configurations adhere to the security baseline.
• Asset and patch process: Implement a regular review cycle for DNS server configurations to identify and address any misconfigurations.

7. Risks, Side Effects, and Roll Back

Disabling or restricting access to DNS over TLS may impact clients that rely on encrypted DNS resolution.

• Risk or side effect 1: Disabling DNS over TLS can break functionality for clients requiring it.
• Risk or side effect 2: Incorrect firewall rules could block legitimate internal DNS traffic.
• Roll back: Restore the original DNS server configuration file and restart the service.

8. References and Resources

• Vendor advisory or bulletin: Check your DNS software vendor’s website for specific guidance on securing TLS connections.
• NVD or CVE entry: No specific CVE exists for running DNS-over-TLS, but related misconfiguration vulnerabilities may be listed on the NVD.
• Product or platform documentation relevant to the fix: https://en.wikipedia.org/wiki/DNS_over_TLS