How to remediate – DNS Server DNSSEC Aware Resolver

1. Introduction

The remote DNS resolver is DNSSEC-aware. This means it can handle DNS records secured with DNSSEC, which helps verify the authenticity of domain names and prevent attacks like DNS cache poisoning. Systems affected are typically those running a public-facing or internal DNS service. A successful attack could lead to reduced trust in DNS responses, potentially impacting confidentiality, integrity, and availability of services relying on DNS resolution.

2. Technical Explanation

The vulnerability exists because the remote DNS resolver accepts DNSSEC options during queries. While not inherently malicious, this indicates a potential for verification if configured to trust keys from zones. An attacker could potentially manipulate DNS responses if the resolver is misconfigured or trusts compromised keys. There are no known CVEs associated with simply being DNSSEC-aware; however, related vulnerabilities exist when combined with improper configuration. For example, an attacker might attempt to poison a cache using crafted DNSSEC records if the server isn’t properly validating signatures.

• Root cause: The DNS resolver supports DNSSEC options.
• Exploit mechanism: An attacker could exploit this by attempting to inject malicious DNS records into the cache, relying on misconfiguration or compromised trust anchors.
• Scope: All systems running a DNS resolver that supports DNSSEC.

3. Detection and Assessment

To confirm whether your system is vulnerable, check if it responds to queries with DNSSEC options enabled. A thorough method involves examining the DNS server configuration for trust anchor settings.

• Quick checks: Use dig +dnssec example.com. If the `AD` flag is present in the answer section, the resolver is DNSSEC-aware.
• Scanning: Nessus plugin ID 139280 (DNSSEC Support) can identify DNSSEC support. This is an example only and may require updates.
• Logs and evidence: Check DNS server logs for messages related to DNSSEC validation or trust anchor loading. The specific log file location varies by operating system and DNS software.

dig +dnssec example.com

4. Solution / Remediation Steps

The following steps outline how to assess and potentially mitigate the risk associated with a DNSSEC-aware resolver.

4.1 Preparation

• Ensure you have access to the DNS server’s configuration files and understand how to modify them. A rollback plan involves restoring from backup.
• A change window may be required, depending on the size of your network and criticality of the DNS service. Approval from a senior administrator might be needed.

4.2 Implementation

1. Step 1: Review your DNS server configuration for trust anchor settings. Ensure only trusted keys are configured.
2. Step 2: If you do not require DNSSEC validation, disable it in the DNS server configuration. This is the most secure option if DNSSEC isn’t actively used.
3. Step 3: Restart the DNS service to apply the changes.

4.3 Config or Code Example

Before

options {
  dnssec-validation auto; // Enable DNSSEC validation
};

After

options {
  dnssec-validation no; // Disable DNSSEC validation
};

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help mitigate the risks associated with a DNSSEC-aware resolver.

• Least privilege: Restrict access to DNS server configuration files to authorized personnel only, reducing the impact if compromised.
• Secure defaults: Configure DNS servers with secure default settings, disabling unnecessary features like DNSSEC validation unless explicitly required.

4.5 Automation (Optional)

Automation is not directly applicable for this vulnerability without knowing your specific DNS server software and configuration management tools.

5. Verification / Validation

• Post-fix check: Run dig +dnssec example.com again. The `AD` flag should no longer be present in the answer section if DNSSEC validation has been disabled.
• Re-test: Re-run the initial detection method (dig +dnssec example.com) to confirm that the issue is resolved.
• Smoke test: Verify that basic DNS resolution still works by resolving a known domain name (e.g., ping google.com).

dig +dnssec example.com

6. Preventive Measures and Monitoring

Preventive measures include regularly reviewing DNS server configurations and keeping software up-to-date.

• Baselines: Update your security baseline to reflect secure DNS configuration settings, including disabling unnecessary features like DNSSEC validation.
• Pipelines: Implement automated checks in CI/CD pipelines to ensure that DNS server configurations adhere to the established baseline.
• Asset and patch process: Establish a regular patch cycle for DNS server software to address known vulnerabilities promptly.

7. Risks, Side Effects, and Roll Back

Disabling DNSSEC validation may prevent you from using DNSSEC-protected zones if they are required. If issues arise, restore the original configuration from backup.

• Risk or side effect 1: Disabling DNSSEC validation might break functionality for services relying on DNSSEC.
• Risk or side effect 2: Incorrectly configured trust anchors could lead to cache poisoning attacks.

8. References and Resources

Links only to sources that match this exact vulnerability.

• Vendor advisory or bulletin: Not applicable for a general awareness issue.
• NVD or CVE entry: Not applicable for a general awareness issue.
• Product or platform documentation relevant to the fix: Consult your DNS server software’s documentation for instructions on configuring DNSSEC validation (e.g., BIND documentation, Microsoft DNS Server documentation).