Subdomain Takeovers and How to Prevent Them

A subdomain takeover is a security vulnerability that occurs when an attacker gains control over a subdomain of a legitimate website due to misconfigurations or abandoned DNS records. This type of attack can have serious consequences, including phishing, malware distribution, data theft, and brand reputation damage. Subdomain takeovers are particularly dangerous because they allow an attacker to host malicious content under a domain that appears trustworthy, making it easier to deceive users into entering sensitive information or downloading harmful files. As many organizations use third-party services to host content, applications, and APIs, the risk of subdomain takeovers has increased, requiring proactive security measures to prevent them.

The core mechanism behind a subdomain takeover involves orphaned or dangling DNS records, which are records that point to external services that are no longer in use. Many organizations use cloud platforms, content delivery networks, and software-as-a-service providers to manage their online infrastructure, often creating CNAME or A records that direct subdomains to third-party services. When these services are decommissioned, unclaimed, or left without proper removal of their corresponding DNS records, attackers can register the abandoned resource and take control of the subdomain. For example, if a company had a subdomain such as app.example.com pointing to a cloud hosting provider but later discontinued the application without deleting the DNS entry, an attacker could register the cloud service and effectively hijack the subdomain.

Attackers actively search for vulnerable subdomains using automated tools that scan for misconfigured or expired DNS records. Once a potential target is identified, they attempt to claim the associated cloud or hosting service to gain control of the subdomain. Since the DNS record remains active, users and automated systems that attempt to access the subdomain will unknowingly interact with the attacker-controlled environment. This opens the door for a variety of malicious activities, including serving fake login pages for credential harvesting, embedding malware in downloadable files, redirecting users to fraudulent sites, or conducting sophisticated social engineering attacks.

One of the primary reasons subdomain takeovers are a persistent issue is the lack of DNS hygiene and lifecycle management in many organizations. Large enterprises often have complex DNS infrastructures with thousands of records, many of which are outdated or no longer actively monitored. Development teams frequently create temporary subdomains for testing and staging environments, but these are not always properly decommissioned when projects end. Additionally, organizations that use third-party services may forget to remove associated DNS records when they migrate to a different provider, leaving dormant subdomains exposed to takeover risks.

Preventing subdomain takeovers requires a systematic approach to DNS management and security. Organizations must conduct regular audits of their DNS records to identify and remove any unused or obsolete subdomains. This includes monitoring all active CNAME, A, and NS records to ensure they still point to valid and actively managed services. Automated scanning tools can assist in identifying orphaned records and flagging potential takeover risks before they can be exploited by attackers.

Implementing domain access controls and restricting DNS management permissions can also reduce the likelihood of misconfigurations that lead to subdomain takeovers. Only authorized personnel should have the ability to create, modify, or delete DNS records, and changes should be logged and reviewed periodically. Using multi-factor authentication for DNS management accounts and integrating DNS security monitoring tools can further strengthen an organization’s defenses against unauthorized modifications.

Another effective strategy is the use of wildcard DNS records and catch-all responses for unclaimed subdomains. By configuring a wildcard entry to direct all undefined subdomains to a controlled environment, organizations can prevent attackers from hijacking specific subdomains that may have been overlooked. This approach ensures that even if a DNS record remains in place, it does not lead to an attacker-controlled resource.

For organizations using cloud services and third-party platforms, it is essential to implement strict deprovisioning procedures when discontinuing a service. When an application or platform is no longer needed, both the service itself and the corresponding DNS records should be deleted promptly. Service providers should also be chosen carefully, with consideration given to whether they offer protections against domain hijacking and subdomain takeovers. Some cloud providers have introduced security measures to prevent unauthorized claims of abandoned resources, but organizations should not rely solely on these measures and must actively manage their DNS configurations.

Raising awareness about subdomain takeovers among IT teams and developers is also crucial for mitigating risks. Many subdomain takeover incidents occur due to a lack of knowledge about the importance of proper DNS management. Providing training on DNS security best practices and incorporating security checks into the development and deployment process can help prevent accidental exposure to takeover risks. Security teams should collaborate with developers to ensure that DNS records are reviewed as part of the project lifecycle, from initial creation to eventual decommissioning.

In addition to internal security measures, organizations should leverage external monitoring solutions to detect signs of subdomain takeovers in real time. Security researchers and ethical hackers often use techniques such as certificate transparency monitoring, passive DNS analysis, and domain reputation tracking to identify suspicious changes in domain infrastructure. By integrating similar techniques into their own security operations, organizations can detect and respond to potential threats before they are exploited by malicious actors.

As cyber threats continue to evolve, subdomain takeovers remain a significant risk that requires ongoing vigilance. Attackers are constantly scanning for opportunities to exploit abandoned or misconfigured DNS records, and even a single overlooked subdomain can serve as an entry point for large-scale attacks. By implementing a comprehensive DNS security strategy that includes regular audits, strict access controls, automated monitoring, and proactive deprovisioning practices, organizations can significantly reduce their exposure to subdomain takeovers and protect their online presence from exploitation. Ensuring that all components of DNS infrastructure are actively maintained and securely managed is not only a technical necessity but a fundamental aspect of overall cybersecurity resilience.

A subdomain takeover is a security vulnerability that occurs when an attacker gains control over a subdomain of a legitimate website due to misconfigurations or abandoned DNS records. This type of attack can have serious consequences, including phishing, malware distribution, data theft, and brand reputation damage. Subdomain takeovers are particularly dangerous because they allow an…