Identification of subdomain takeover

Subdomain takeover is a security vulnerability that occurs when an attacker gains control over a subdomain of a target website. This can happen when a subdomain points to a service (like a cloud host, web service, or CDN) that has been deprovisioned or is no longer in use, but the DNS record for the subdomain remains in place. If the attacker can claim or register the target service account, they can control content served from the subdomain.

Common Indicators of Subdomain Takeover:

  1. DNS Pointing to Inactive Services: The DNS records for a subdomain point to a service that is no longer active or has been deprovisioned.
  2. Orphaned DNS Records: Subdomains with DNS records that are not in use or no longer associated with a valid, active service endpoint.
  3. Generic Error Messages: Accessing the subdomain displays generic service provider error messages indicating the resource is not available or not in use.
  4. Service Provider Notifications: Some service providers display explicit notifications that a particular domain or resource is available to be claimed.

How to Identify Subdomain Takeover:

  1. DNS Enumeration: Enumerate all DNS entries for a domain to identify subdomains and check where they point to. Tools like subdomain enumeration tools can automate this process.
  2. Service Verification: For each subdomain, verify if the service it points to is active and properly configured. If the service is inactive or the endpoint is not in use, it may be vulnerable.
  3. Automated Scanning Tools: Use automated tools that can scan for subdomains vulnerable to takeover by checking for inactive services or misconfigurations.
  4. Manual Inspection: Manually inspect subdomains for signs of takeover vulnerability, such as default service provider pages or error messages indicating that the site is unclaimed.

Mitigation Strategies:

  1. Regular DNS Audits: Conduct regular audits of your DNS records to ensure that all subdomains are accounted for and pointing to active, intended services.
  2. Remove Unused Records: If a service is deprovisioned or no longer in use, promptly update or remove the corresponding DNS records to prevent them from being hijacked.
  3. Monitor Subdomain Integrity: Implement monitoring to detect changes in subdomain content or configuration, which can indicate unauthorized takeover.
  4. Use Subdomain Verification: Some services offer verification mechanisms to ensure that the entity claiming a subdomain has the right to do so. Utilize these features when available.
  5. Employ DNS Security Features: Use DNS security extensions and features like DNSSEC to enhance the integrity and trustworthiness of DNS records.

Addressing subdomain takeover vulnerabilities is crucial for protecting the integrity of a domain's namespace and preventing attackers from exploiting these subdomains to conduct phishing attacks, distribute malware, or tarnish an organization's reputation.