For those using Microsofts Cloud Computing services, the Azurewebsites.net domain would be a familiar one. It is usually used for staging servers and usually contains (on a subdomain) a complete copy of your website. Unfortunately, many devs simply leave this environment open to indexation:
There are literally thousands of sites for that query. Unfortunately, apart from the obvious SEO risk (content duplication, staging outranking primary site) this also exposes businesses to significant negative threats via subdomain takeovers.
What Are Subdomain Takeovers?
Subdomain takeovers represent a critical and often overlooked cybersecurity risk. As businesses and individuals alike expand their online presence, they also inadvertently increase their vulnerability to various cyber threats. One such threat is the subdomain takeover, a scenario where a malicious actor gains control over a company’s subdomain, redirecting it to a server they control. Lets explore the concept of subdomain takeovers, the specific risks they pose, particularly in the context of Azure and other cloud services, and how organizations can protect themselves against such threats.
Understanding Subdomain Takeovers
Subdomain takeovers occur when a DNS entry points to a resource that is no longer active. For example, a company might have a subdomain like blog.company.com pointing to a content management system (CMS) hosted on a cloud provider. If the company discontinues the CMS service but fails to remove the DNS entry pointing to that now-defunct service, the subdomain becomes vulnerable. An attacker can then create an account with the cloud provider, claim the abandoned resource, and effectively take control of the subdomain.
This form of attack is particularly insidious because it often goes unnoticed until significant damage has been done. Once an attacker has control of a subdomain, they can use it to host malicious content, launch phishing attacks, or intercept sensitive data. Given that the subdomain still appears to be legitimate—bearing the company’s URL—users are less likely to question its authenticity, making these attacks highly effective.
The Risks in the Azure Ecosystem
Azure, Microsoft’s cloud computing service, is widely used by businesses for hosting applications, services, and other digital resources. Like other cloud services, Azure relies heavily on DNS for routing traffic to these resources. This dependence on DNS creates an opportunity for subdomain takeovers if resources are decommissioned but their DNS records are not appropriately updated.
In Azure, subdomain takeovers can occur in several ways:
- Azure App Services: Companies often use Azure App Services to host web applications. If an application is deleted, but the DNS record pointing to it remains, an attacker could potentially create a new app service with the same name and take over the subdomain.
- Azure Blob Storage: Similar risks exist with Azure Blob Storage. If a blob storage account is deleted, but the DNS entry for the associated subdomain isn’t removed, an attacker could claim that subdomain by creating a new blob storage account with the same name.
- Azure Cloud Services: The same scenario applies to other Azure cloud services. Any orphaned DNS entry that points to a decommissioned resource could be exploited by a malicious actor.
Real-World Consequences
The potential damage from a subdomain takeover is vast. In addition to the direct impact on an organization’s reputation, there are also significant security risks. Once in control of a subdomain, an attacker can host malicious scripts, which can be used to steal data from visitors. For example, a subdomain takeover could be used to serve a phishing page that looks identical to the company’s legitimate login page, tricking users into entering their credentials.
Moreover, because the subdomain is still under the company’s main domain, it may have a valid SSL certificate, further lending credibility to the fake site. This makes it easier for attackers to deceive victims and carry out more sophisticated attacks.
Another significant risk is the potential for these subdomains to be used as part of a larger attack infrastructure. For instance, attackers could use the subdomain as a command-and-control (C&C) server, coordinating attacks on other systems. The legitimate appearance of the subdomain makes it more likely that these malicious activities will go undetected.
Preventing Subdomain Takeovers
Preventing subdomain takeovers requires a proactive approach to DNS management and resource decommissioning. Below are some key strategies organizations can implement to protect themselves:
- Regular Audits of DNS Records: Organizations should regularly audit their DNS records to ensure that all entries point to active resources. Any outdated or unused DNS entries should be removed immediately to prevent them from being hijacked.
- Use of Azure DNS Alias Records: Azure DNS alias records are a powerful tool in preventing subdomain takeovers. These records link a DNS entry directly to an Azure resource, ensuring that if the resource is deleted, the DNS record is automatically updated or invalidated. This reduces the risk of leaving behind orphaned DNS records that could be exploited.
- Custom Domain Verification: Azure App Services allows for custom domain verification, which ensures that only the rightful owner of a domain can associate it with an Azure service. This adds an extra layer of security by preventing unauthorized users from linking decommissioned subdomains to their own resources.
- Monitoring and Alerts: Organizations should implement monitoring systems that alert them when changes are made to their DNS records. By staying vigilant, they can quickly identify and address any unauthorized modifications.
- Employee Training: Human error often plays a role in subdomain takeovers, whether through neglecting to remove DNS entries or failing to follow proper decommissioning procedures. Regular training and awareness programs can help employees understand the importance of maintaining up-to-date DNS records and the risks associated with subdomain takeovers.
- Engaging with Cloud Security Tools: Leveraging tools like Microsoft Defender for App Service can provide additional protection. These tools can scan for vulnerabilities and offer recommendations for securing DNS records and other potential points of entry.
Responding to a Subdomain Takeover
In the unfortunate event that a subdomain takeover occurs, swift action is critical. The first step is to regain control of the subdomain. This typically involves updating or deleting the DNS record associated with the compromised subdomain. If the attacker has associated the subdomain with their own resource, it may be necessary to work with the cloud provider to reclaim it.
Once control is restored, it’s important to conduct a thorough investigation to determine the extent of the damage. This includes identifying any data that may have been compromised and notifying affected users. Organizations should also review their DNS management processes to prevent future takeovers.
The Broader Implications for Cybersecurity
Subdomain takeovers are just one example of the broader risks associated with poor DNS management. As organizations increasingly rely on cloud services, the importance of maintaining strict control over DNS records cannot be overstated. DNS plays a critical role in routing traffic and securing digital assets, and any lapse in its management can have severe consequences.
Furthermore, the rise of subdomain takeovers highlights the need for better integration between cloud services and DNS management tools. By making it easier to manage and monitor DNS records, cloud providers can help reduce the risk of subdomain takeovers and other DNS-related vulnerabilities.
Conclusion
Subdomain takeovers are a serious threat that can have far-reaching consequences for businesses and their customers. By understanding the risks and implementing robust DNS management practices, organizations can protect themselves against this form of attack. As the digital landscape continues to evolve, staying ahead of these threats will require vigilance, proactive security measures, and ongoing education.
In the end, the key to preventing subdomain takeovers lies in maintaining a strong, security-focused culture within the organization, where every employee understands the importance of protecting the company’s digital assets.