Subdomain Takeovers: Understanding the Threat and Preventing It

Subdomain takeovers are a significant and often overlooked vulnerability in the domain industry. This type of attack occurs when a domain’s subdomain, which is pointed to an external service, becomes unclaimed or inactive, allowing an attacker to hijack it. The results can range from defacement and phishing attacks to data breaches and reputational damage. As more organizations integrate third-party services and rely on dynamic hosting, the risks associated with subdomain takeovers have grown, making it essential to understand how these takeovers happen and what steps can be taken to prevent them.

At the core of a subdomain takeover is the misconfiguration or decommissioning of external services associated with a subdomain. Businesses frequently use subdomains to point to external platforms, such as content delivery networks (CDNs), cloud services, or third-party applications like GitHub Pages or Amazon S3. This allows organizations to distribute content or services across different platforms seamlessly. However, problems arise when a subdomain is no longer actively connected to its designated service. For example, if a company discontinues the use of a particular service but forgets to remove the DNS record pointing to it, the subdomain becomes vulnerable. The external service might release the subdomain, making it available for registration or control by another user. An attacker, aware of the vulnerability, can claim ownership of the abandoned service and, by extension, control the subdomain.

Once an attacker takes control of the subdomain, they can use it for various malicious purposes. One common use is phishing. Since the subdomain is associated with a legitimate domain, users who visit the hijacked subdomain may not suspect anything is wrong. The attacker can create a phishing site under the compromised subdomain to steal sensitive information such as login credentials or payment details. Because the domain itself remains trusted, many security systems and end-users might not recognize the threat, increasing the effectiveness of the attack.

Another way subdomain takeovers can cause damage is through brand defacement or hosting malicious content. Attackers can use the hijacked subdomain to host malware or run scams, tarnishing the reputation of the legitimate domain owner. Furthermore, this kind of attack can lead to search engine penalties or warnings from browsers, which may identify the compromised subdomain as malicious. These incidents can severely affect the organization’s credibility, especially if customers or partners are tricked into interacting with fraudulent or harmful content.

The technical mechanism of subdomain takeovers is straightforward. When a DNS record is created to point a subdomain to an external service, it tells the DNS system to direct traffic from that subdomain to the external service’s servers. However, if the external service is decommissioned or deleted without the DNS record being updated, the subdomain remains pointed to a service that no longer exists. The external platform may allow anyone to claim the service at that subdomain if it’s not in use, giving attackers the opportunity to take control by registering the service with the abandoned subdomain.

Subdomain takeovers are not limited to obscure or little-used services. Major platforms like GitHub Pages, Heroku, and Amazon S3 have been exploited in this manner, highlighting the widespread nature of this issue. In some cases, companies have had large numbers of subdomains taken over, leading to prolonged and damaging consequences. Attackers who control a subdomain can alter DNS settings, inject malicious scripts, and misuse SSL certificates, which further legitimizes their control and gives them the power to manipulate trust in ways that can go undetected by both end-users and security tools for extended periods.

Preventing subdomain takeovers is essential, particularly for organizations that rely heavily on external services and cloud infrastructure. One of the most effective ways to mitigate this threat is to maintain strict oversight of DNS records. DNS hygiene involves regularly auditing DNS records to ensure that all subdomains are correctly mapped to active services. This process should include identifying unused or abandoned subdomains and promptly removing their DNS records when they are no longer needed. By doing so, companies can eliminate the risk of leaving dormant subdomains open to attack.

In addition to DNS auditing, organizations should closely monitor the services to which their subdomains are linked. When decommissioning an external service, it is crucial to remove the associated DNS entries at the same time. This helps prevent any gaps in ownership that attackers could exploit. Companies should also implement clear procedures for updating DNS records as part of their decommissioning processes, ensuring that changes in infrastructure are reflected in DNS management.

Some registrars and cloud providers offer tools that can alert administrators to potential vulnerabilities in subdomain configuration. These tools can flag inactive DNS records or identify subdomains that are pointing to expired services, enabling organizations to take corrective action before an attacker has the opportunity to exploit the weakness. Integrating these kinds of monitoring tools into the domain management process can significantly reduce the risk of subdomain takeovers.

Another layer of defense is implementing CNAME flattening or other DNS configurations that minimize the reliance on specific third-party services. By carefully controlling how DNS records are set up and using more resilient DNS management techniques, organizations can reduce the chances of inadvertently leaving subdomains vulnerable to takeover.

A security-conscious development process can also help prevent subdomain takeovers. Developers and IT teams should be educated about the risks and be encouraged to design systems that reduce the number of dependencies on external services. This can involve consolidating services into fewer, more secure platforms, or ensuring that fallback mechanisms are in place for when services are discontinued.

Additionally, penetration testing and regular security assessments should include checks for vulnerable subdomains. Security teams can use tools to scan for inactive DNS entries or services that are no longer in use, giving organizations an opportunity to fix potential issues before they are exploited. In some cases, organizations may want to employ external security firms to conduct audits of their DNS infrastructure and identify areas of risk.

The consequences of a subdomain takeover can be severe, both in terms of financial damage and reputational harm. However, with vigilant DNS management, proactive monitoring, and strong operational practices, these attacks can be prevented. Subdomain takeovers remain a significant threat in today’s increasingly decentralized and service-oriented internet landscape, but by adopting a comprehensive security approach, organizations can significantly reduce the risks. As digital infrastructures continue to grow more complex, maintaining a sharp focus on DNS security will be key to protecting businesses and users from this evolving threat.

Subdomain takeovers are a significant and often overlooked vulnerability in the domain industry. This type of attack occurs when a domain’s subdomain, which is pointed to an external service, becomes unclaimed or inactive, allowing an attacker to hijack it. The results can range from defacement and phishing attacks to data breaches and reputational damage. As…