Architectural Insights into Preventing Subdomain Takeovers

Dhruv | Jul 15, 2024

Subdomain takeovers are a significant security risk that occurs when a malicious actor gains control of a subdomain previously associated with an external service. This vulnerability often arises when DNS records remain active after the service has been decommissioned, creating a security gap. This blog post explores the architectural aspects of subdomain takeovers and provides insights on how both consumers and Cloud Service Providers (CSPs) can mitigate these risks.

What is a Subdomain Takeover?

A subdomain takeover happens when a subdomain pointing to an external service remains active after the service is no longer in use. Attackers exploit this gap to gain control of the subdomain, which can then be used to host malicious content or conduct phishing attacks under the guise of a trusted domain.

Vulnerabilities Leading to Subdomain Takeovers

DNS Configuration and Orphaned Records

One major issue is the presence of orphaned DNS records. When a service is decommissioned, its DNS records should also be removed. However, this step is often overlooked, leaving these records dangling and exploitable.

Cloud Service Integrations

Many subdomain takeovers occur due to misconfigured cloud services. For example, subdomains pointing to resources on AWS S3, Azure, or Heroku can become vulnerable if the services are deleted without proper DNS cleanup.

Third-Party Service Dependencies

Relying on third-party services can introduce risk if those services are not correctly decommissioned. Subdomains pointing to unused third-party services can be hijacked if the DNS records remain unchanged.

Preventing subdomain takeovers

Cloud Consumer

Automated DNS Cleanup

Consumers should implement automation to ensure DNS records are updated or removed in tandem with service decommissioning. This reduces human error and keeps DNS records clean. Tools like AWS Route 53 and Azure DNS offer automated management capabilities that can help streamline this process.

Service Deprovisioning Protocols:

Develop and enforce strict protocols for decommissioning services. Ensure that DNS records are part of the decommissioning checklist. This might involve creating internal policies and checklists to ensure all associated resources, including DNS records, are cleaned up when a service is no longer needed.

Continuous Monitoring and Auditing:

Regularly audit DNS records and monitor for any anomalies. Use tools that can alert you to potential vulnerabilities in real-time. Services like DNS monitoring tools can help track changes and alert you to any potential security issues.

Role-Based Access Control:

Implement role-based access control to limit who can create and manage DNS records. This reduces the likelihood of misconfigurations. Ensure only authorized personnel have access to critical DNS configurations and regularly review access permissions.

What changes can Cloud Service Providers implement?

Validating Domain Ownership:

CSPs should ensure that domain ownership is validated before any DNS changes are allowed. This can prevent unauthorized modifications that might lead to vulnerabilities. Implementing mandatory domain validation processes can help secure the DNS infrastructure.

Adding Entropy to Records:

Introduce randomness or entropy in DNS records to make them less predictable and harder for attackers to exploit. This involves creating more complex and less predictable subdomain names, which can significantly reduce the risk of automated attacks.

Preventing Re-registration of Deleted Domains:

Prevent previously deleted domains from being re-registered to avoid potential takeovers. CSPs can implement policies that prevent the immediate reuse of recently deleted subdomains, ensuring that any previous configurations are fully removed and cannot be exploited.

Conclusion

Addressing architectural vulnerabilities is crucial in preventing subdomain takeovers. Consumers should focus on automated DNS cleanup, strict service decommissioning protocols, continuous monitoring, and role-based access control. On the other hand, CSPs need to validate domain ownership, add entropy to records, and prevent re-registration of deleted domains. By implementing these preventive measures, both consumers and CSPs can significantly reduce the risk of these vulnerabilities. Understanding and addressing these aspects is key to maintaining a secure environment. Stay safe and secure!

References and more reading!