Dangling DNS Records and Subdomain Takeovers: Protecting Your Domain When Switching Cloud Services

Article summary: Canceling a cloud service or marketing tool can leave behind a dangling DNS record that still points to the old provider. Attackers can exploit these records to host scam pages, malware or other content under a legitimate business domain. Subdomain takeover prevention removes these forgotten connections before they can become a security risk.

Switching software is supposed to be the easy part. Cancel the old subscription, sign up for the new one and move on.

The domain record left pointing at that cancelled service does not clean itself up. 

A short domain health check built into every migration catches the kind of gap that a stranger could otherwise claim as their own which is exactly what subdomain takeover prevention is about.

What Happens When a DNS Record Outlives Its Service

DNS (Domain Name System) acts like a directory for the internet by directing browsers and other services to the right destination when someone visits your website or a subdomain such as promo.yourbusiness.com.

Many cloud services, website builders, help desks and marketing platforms let businesses connect a subdomain using a CNAME record. The CNAME directs traffic from your subdomain to a hostname managed by the third-party provider.

That setup works as intended as long as the underlying service or resource remains active.

The problem begins when that resource is deleted or decommissioned but the DNS record remains. The CNAME may continue pointing to a resource that no longer exists which creates what is known as a dangling DNS record. In some cases, that leftover record can create an opportunity for someone else to claim the underlying resource and potentially take control of the subdomain.

How a Dangling DNS Record Becomes a Subdomain Takeover

The DNS Record Stays Behind

When your business stops using a cloud service or online platform, the DNS record that connected your subdomain to that service does not necessarily disappear with it.

If nobody removes the record, it can continue pointing to a service or resource your business no longer uses. This is known as a dangling DNS record.

Someone Else May Be Able to Claim the Old Resource

The real danger appears when the provider allows someone else to claim the resource your DNS record still points to.

Microsoft warns that when a DNS record points to a deprovisioned cloud resource, an attacker may be able to take advantage of that connection and serve content through the company’s subdomain.

That means a web address your customers already recognize and trust could potentially lead them to content controlled by someone else.

Microsoft recommends removing DNS records associated with resources you no longer use and making DNS cleanup part of the process whenever cloud services are retired.

It is a Known Security Problem

The OWASP Subdomain Takeover Prevention Cheat Sheet notes that subdomain takeover is consistently among the most reported findings in bug bounty programs.

Part of the problem is simple housekeeping. Businesses are good at creating DNS records when they launch new services but those records can easily be forgotten when the services are canceled or replaced.

What an Attacker Can Do With Your Subdomain

If an attacker successfully takes over a forgotten subdomain, they can use an address that still carries your company’s name. That can make whatever they put there look much more legitimate than a random website.

An attacker could use the subdomain to:

• Create a convincing phishing page designed to steal passwords or other sensitive information
• Host scams or malicious content under a web address customers may recognize and trust
• Make phishing attempts appear more legitimate because the link uses your company’s real domain
• Damage your reputation if customers encounter fraudulent or harmful content associated with your business

It is the same trust that makes phishing after a well-known data breach so effective. People are more likely to trust a link when they recognize the company or domain behind it.

Protecting Your Domain During and After a Cloud Migration

Keep a DNS Record Inventory

Keep a current list of your DNS records, what they connect to and who is responsible for them.

That makes it much easier to identify records tied to old websites, cloud services, marketing platforms and other tools that your business no longer uses. OWASP recommends maintaining an inventory that connects DNS records with the resources and services behind them.

Make DNS Cleanup Part of Offboarding

When your business stops using a cloud service, removing or updating its DNS records should be part of the process.

Do not assume that canceling an account or deleting a cloud resource will also clean up the DNS records associated with it. Building DNS cleanup into your standard offboarding process helps prevent forgotten records from lingering long after a service is gone.

Check for Forgotten Records Regularly

Even with a good offboarding process, old DNS records can slip through the cracks.

OWASP recommends regularly scanning DNS records for entries pointing to resources that no longer exist. For organizations using automated monitoring, OWASP suggests running these checks daily or weekly.

For a small business, the important part is making DNS reviews routine rather than waiting until a forgotten record becomes a security problem.

Migrating Cloud Services Soon?

A cloud migration is a good opportunity to review your DNS records and make sure old services are not leaving forgotten connections behind.

Sound Computers can review your current DNS records, identify entries connected to outdated or unfamiliar services and make DNS cleanup part of your ongoing IT consulting and cloud migration process.

Contact Sound Computers to schedule a consultation. Call (860) 577-8060, reach us online or email info@soundcomputers.net.

Article FAQs

What is a dangling DNS record?

A dangling DNS record is a DNS entry (often a CNAME) that still points to a cloud resource or third-party service that has been deleted, canceled or is no longer in use. The DNS record remains even though the resource it was created for is gone.

How is a subdomain takeover different from a normal hack?

Many cyberattacks involve stealing credentials, exploiting software vulnerabilities or gaining unauthorized access to a company’s systems. A subdomain takeover can happen differently. If an old DNS record points to an abandoned resource that someone else can claim, an attacker may be able to use that connection without first breaking into the company’s network.

Does this only affect large companies?

Yes. Any business that connects subdomains to cloud platforms, website tools, help desks, marketing services or other third-party providers can potentially end up with dangling DNS records. The risk appears when those services are retired but the related DNS records are left behind.