A subdomain takeover occurs when an attacker gains control over a subdomain of a target domain. This typically happens when the subdomain is configured in the Domain Name System (DNS) but lacks an active virtual host to serve content. In such cases, attackers can exploit the misconfiguration by setting up their own virtual host and hosting content on the subdomain.

This vulnerability can arise when cloud-hosted web services are incompletely decommissioned. For instance, if a web project hosted on a cloud provider is deleted but the DNS entries pointing to it are not removed, an attacker can re-register the subdomain at the cloud provider and control the content hosted there. Proper management of DNS records and virtual hosts is crucial to prevent such vulnerabilities.

How does a Subdomain Takeover Work?

Subdomain take overs work by exploiting DNS misconfigurations. When a subdomain has a DNS entry, such as a CNAME record, pointing to a cloud service that no longer hosts content, it becomes vulnerable. Attackers can identify these subdomains through DNS scanning and monitoring for entries that resolve to inactive hosts.

Once a vulnerable subdomain is identified, the attacker can re-register the host at the cloud provider. By setting up a virtual host with the same subdomain name, they effectively take control of the subdomain. This allows them to host their own content, which can be used for malicious purposes.

The process involves either provisioning a virtual host before the legitimate owner or exploiting deprovisioned hosts that still have active DNS entries. Proper lifecycle management of DNS records and virtual hosts is essential to prevent such takeovers.

What are Examples of Subdomain Takeovers?

Examples of subdomain takeovers highlight the widespread nature of this vulnerability. In 2014, Frans Rosén from Detectify discovered that 17 service providers were susceptible to subdomain takeovers. This initial finding led to further research, identifying over 100 services with similar vulnerabilities. These discoveries underscored the importance of vigilant DNS management and the risks associated with incomplete decommissioning of cloud-hosted services.

Another notable example involves third-party services like GitHub. Attackers can claim a deleted username and replace hosted resources, such as JavaScript files, with malicious code. This method demonstrates the diverse tactics attackers use to exploit subdomain takeovers, emphasizing the need for comprehensive monitoring and timely removal of unused DNS entries.

What are the Potential Risks of Subdomain Takeovers?

The potential risks of subdomain takeovers are significant and multifaceted. Here are some of the key risks associated with this vulnerability:

• Data Breaches: Attackers can access sensitive information such as session tokens and user data by exploiting cookies set from the main domain.

• Unauthorized Access: Malicious actors can bypass content security policies to capture protected information, including login credentials and other sensitive data.

• Reputation Damage: Hosting malicious content on a compromised subdomain can severely damage the trust and reputation of the main domain.

• Financial Losses: The potential for malicious activities on the compromised subdomain can lead to significant financial damage.

• Service Disruptions: Subdomain takeovers can disrupt normal operations, leading to service interruptions and affecting user experience.

How can you Protect Against Subdomain Takeovers?

Protecting against subdomain takeovers requires a proactive and comprehensive approach. Here are some key strategies to safeguard your subdomains:

• Regularly Audit DNS Records: Conduct frequent audits to ensure no obsolete or unused DNS entries exist, reducing the risk of exploitation.

• Implement Lifecycle Management: Establish clear processes for provisioning and deprovisioning hosts, ensuring DNS records are created last and removed first.

• Maintain an Inventory: Keep an updated inventory of all domains and their hosting providers to monitor and manage them effectively.

• Monitor Subdomain Activity: Regularly monitor and test subdomains to detect any unauthorized changes or vulnerabilities promptly.

• Coordinate Across Departments: Ensure all teams involved in DNS and hosting management are aligned to avoid misconfigurations and lapses in security.