How to remediate – DNS Server Zone Transfer Information Disclosure (AXFR)

1. Introduction

The DNS Server Zone Transfer Information Disclosure (AXFR) vulnerability allows remote attackers to obtain a list of potential targets by performing zone transfers on a misconfigured name server. This information can be used to map the network topology and identify servers with specific applications, such as proxy or payroll systems. A successful exploit could lead to reconnaissance and potentially further attacks. Confidentiality is most at risk.

2. Technical Explanation

The vulnerability occurs when a DNS server allows unrestricted zone transfers. Zone transfers are intended for legitimate purposes like synchronizing secondary DNS servers, but they expose the entire DNS record set to anyone who requests it. Attackers can use tools like dig or nslookup with the AXFR option to request and receive this information. CVE-1999-0532 describes this issue.

• Root cause: The DNS server is configured to allow zone transfers from any source IP address, rather than restricting access to authorized servers.
• Exploit mechanism: An attacker sends a query to the DNS server requesting a zone transfer for a specific domain. If allowed, the server responds with all records in that zone. For example, an attacker could use dig axfr example.com @dns.example.com.
• Scope: This vulnerability affects DNS servers running on various platforms and software versions if not properly configured.

3. Detection and Assessment

You can confirm whether a system is vulnerable by checking its zone transfer settings and attempting to perform an unauthorized zone transfer.

• Quick checks: Use nslookup or dig to query the DNS server for a zone transfer.
• Scanning: Nessus plugin ID 10853 can detect open AXFR transfers. This is provided as an example only.
• Logs and evidence: Check DNS server logs for requests related to zone transfers (AXFR queries). The log location varies depending on the DNS server software used, but common paths include /var/log/syslog or event logs in Windows.

dig axfr example.com @dns.example.com

4. Solution / Remediation Steps

Limit DNS zone transfers to only the servers that require the information.

4.1 Preparation

• Ensure you have access credentials for the DNS server management interface. A roll back plan is to restore the backed-up configuration file.
• Changes should be made during a planned maintenance window with appropriate approvals.

4.2 Implementation

1. Step 1: Identify authorized secondary DNS servers that legitimately require zone transfers.
2. Step 2: Configure the DNS server to allow zone transfers only from these authorized IP addresses or networks. The specific configuration method varies depending on the DNS server software (e.g., BIND, Windows DNS).
3. Step 3: Restart the DNS service to apply the changes.

4.3 Config or Code Example

Before

// BIND configuration file (named.conf.options)
allow-transfer { any; };

After

// BIND configuration file (named.conf.options)
allow-transfer { 192.168.1.0/24; 10.0.0.1; }; // Replace with authorized IPs

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this issue.

• Practice 1: Least privilege – restrict access to DNS zone transfers only to servers that need the information, reducing the potential impact if exploited.
• Practice 2: Secure defaults – ensure default configurations do not allow unrestricted zone transfers.

4.5 Automation (Optional)

Configuration management tools like Ansible can automate this change across multiple DNS servers.

---
- hosts: dns_servers
  tasks:
    - name: Restrict zone transfers in BIND configuration
      lineinfile:
        path: /etc/named.conf.options
        regexp: '^allow-transfer { any; }'
        line: 'allow-transfer { 192.168.1.0/24; };' # Replace with authorized IPs
      notify: Restart named service
  handlers:
    - name: Restart named service
      service:
        name: named
        state: restarted

5. Verification / Validation

• Post-fix check: Run dig axfr example.com @dns.example.com and verify that it returns a “REFUSED” or similar error message, indicating that the zone transfer is no longer allowed from your IP address.
• Re-test: Repeat the detection method (from section 3) to confirm that the vulnerability is resolved.
• Smoke test: Verify that legitimate DNS queries for domain names within the zone still resolve correctly.
• Monitoring: Monitor DNS server logs for any unauthorized AXFR requests, which could indicate an attempt to bypass the restrictions.

dig axfr example.com @dns.example.com

6. Preventive Measures and Monitoring

Implement preventive measures and monitoring to reduce the risk of future zone transfer vulnerabilities.

• Baselines: Update your security baseline or policy to include a requirement for restricting DNS zone transfers.
• Pipelines: Include checks in your CI/CD pipeline to ensure that DNS server configurations comply with security standards, such as CIS benchmarks.
• Asset and patch process: Review DNS server configurations regularly (e.g., quarterly) to identify and address any misconfigurations.

7. Risks, Side Effects, and Roll Back

Incorrectly configuring zone transfer restrictions could disrupt legitimate DNS services.

• Risk or side effect 1: Restricting access to the wrong IP addresses can prevent secondary DNS servers from synchronizing, leading to DNS resolution failures.
• Risk or side effect 2: Incorrect syntax in the configuration file may cause the DNS service to fail to start.
• Roll back: Restore the backed-up DNS server configuration file and restart the service. If you cannot restore, revert the changes manually by correcting any errors in the configuration file.

8. References and Resources

Links only to sources that match this exact vulnerability.

• Vendor advisory or bulletin: No specific vendor advisory available for CVE-1999-0532, but consult your DNS server software documentation.
• NVD or CVE entry: https://nvd.nist.gov/vuln/detail/CVE-1999-0532
• Product or platform documentation relevant to the fix: BIND Documentation on allow-transfer