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A dangling DNS record can let an attacker serve content through an organization’s subdomain—but only if the hosted resource it points to has been removed and the relevant provider lets someone else claim it. The fix is to treat DNS records and hosted resources as one lifecycle: remove or update the DNS entry when its target is retired, and verify ownership controls for the service involved.
What is a dangling DNS record?
A DNS record is dangling when it still points to a hosted resource that no longer exists or is no longer controlled by the organization. A common case is a CNAME record that directs a subdomain, such as service.example.com, to a cloud or hosting provider. If the service is deleted but the CNAME remains, the organization’s DNS still directs visitors toward a resource name that may be available to someone else.
Microsoft Learn describes subdomain takeovers as a common, high-severity threat for organizations that regularly create and delete many resources. That is a qualitative risk characterization, not a measured estimate of how many major organizations are exposed. A stale record alone does not prove that a takeover is possible: exploitability depends on the provider, the resource’s state, and the provider’s rules for claiming or verifying the name. See Microsoft’s guidance on preventing dangling DNS entries, OWASP’s subdomain takeover testing guide, and the UK NCSC Vulnerability Disclosure Toolkit V2.
How can a dangling record lead to a takeover?
- The organization publishes a DNS record. Often a CNAME, it directs a subdomain to a named resource at a cloud or hosting service.
- The resource is retired, but DNS is not updated. The public record continues to send requests to the old service name.
- The name becomes claimable. If the provider allows another party to register or recreate the relevant resource or namespace, an attacker may claim it.
- The attacker serves content through the organization’s subdomain. Visitors still use the organization’s trusted hostname, even though the underlying service is controlled by someone else.
This sequence is provider-specific. Not every stale CNAME can be claimed, and different services have different resource and ownership rules. OWASP also notes that NS subdomain takeover is less likely but can have especially broad consequences because it may give an attacker control over a DNS zone.
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What can an attacker do with a hijacked subdomain?
The attacker may be able to host content at the organization’s subdomain, making phishing or other abuse appear to come from a familiar, trusted domain. The impact depends on what the subdomain is used for and what the provider allows the attacker to control.
- Impersonate a trusted service: A convincing page on a company subdomain can mislead visitors or employees.
- Abuse application trust: If an affected web application exposes cookies to subdomains, a hijacked subdomain may create a cookie-harvesting risk. This depends on the application’s cookie scope and configuration; it is not an automatic result of every takeover.
- Appear secure in a browser: Microsoft warns that someone who has taken control of a subdomain may be able to obtain a valid SSL certificate for it. HTTPS shows that a connection is encrypted and the certificate is valid for the hostname; by itself, it does not prove the organization still controls the hosted resource.
- Damage trust and reputation: Abuse on an organization-branded hostname can create confusion and undermine confidence in its services.
These are potential consequences, not guaranteed outcomes. The attacker must first be able to claim the resource, and the practical impact depends on the subdomain’s applications, data, and access controls.
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How should administrators find and fix dangling DNS?
1. Tie DNS records to resource owners
Keep an inventory that connects each public DNS record to its target service, an accountable owner, and the resource’s lifecycle. Review the mapping when a service is changed or retired. Microsoft recommends removing records that point to unavailable resources rather than leaving them in DNS.
2. Check provider-specific ownership conditions
Do not rely only on whether a hostname resolves. Determine whether the provider still recognizes the resource, whether another party could claim its name, and whether any required domain verification or reservation control is active. A scanner’s provider-specific signature is a lead to investigate, not conclusive proof that a takeover is possible.
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3. Use tools within their documented scope
- Azure-related CNAMEs: Microsoft provides the
Get-DanglingDnsRecordsPowerShell tool to list domains with CNAME records associated with existing Azure resources in subscriptions or tenants. CNAMEs hosted by other DNS services can be supplied as input when they point to Azure resources. See Microsoft’s dangling DNS guidance and tool instructions. - AWS environments: AWS describes a detection approach using AWS Config and Security Hub. Its sample implementation uses an AWS Config custom rule to report noncompliance to AWS Config and AWS Security Hub, and describes extending inventory across accounts with an AWS Config Aggregator. Review the current sample’s prerequisites before putting it into operation: AWS Security Blog detection guidance and AWS Samples implementation.
4. Enable relevant provider guardrails
For Azure App Service, Microsoft documents hostname reservation and domain verification controls intended to prevent an outside party from creating an app with the same default hostname. Apply the controls that fit the service and hostname in use; they are Azure-specific, not a universal fix for dangling DNS across providers. See Microsoft’s Azure App Service subdomain takeover prevention documentation.
5. Remediate and confirm
Assign an owner to investigate each finding. Remove or correct the DNS record, or restore the intended service resource under organizational control. Then verify that the hostname points to the intended destination and that the retired resource or hostname is no longer available for another party to claim under the provider’s rules.
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How do DNSSEC and takeover prevention fit together?
DNSSEC helps protect DNS data integrity and authenticity; it does not remove a stale record or establish who can claim a deleted hosted resource. Use DNSSEC as a complementary DNS security measure, alongside resource inventories, retirement procedures, and provider-specific hostname ownership controls. NIST covers DNSSEC in its SP 800-81 Rev. 3, Secure Domain Name System (DNS) Deployment Guide.
What should an organization compare when choosing a detection process?
A tool is useful only to the extent that it covers the organization’s actual DNS and hosted services and gets findings to someone who can act. Assess a process against these questions:
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- Does it connect DNS changes to resource creation and decommissioning?
- Which DNS providers, cloud accounts, and hosted services are in scope?
- Does it check service-specific ownership or verification conditions, rather than relying only on DNS resolution?
- How quickly are findings routed to an accountable owner?
- Are relevant provider controls, such as Azure App Service hostname reservation, enabled?
Microsoft’s Azure guidance and AWS’s Config-based approach address different provider environments. Neither should be treated as a universal detector; coverage and operational fit determine whether a process is useful.
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