Dangling DNS Records: An Overlooked Security Risk and How to Eliminate It

What Are Dangling DNS Records?

A dangling DNS record refers to any DNS entry that remains in your domain’s DNS zone even though the target it points to is no longer valid or in use. This situation typically arises due to changes in infrastructure or services that are not accompanied by corresponding DNS updates. Common scenarios include:

• Decommissioned services: For example, you might have a subdomain service.example.com set as a CNAME (alias) pointing to a cloud service (like an Azure or Amazon AWS host). If you later shut down that cloud service or move it elsewhere but forget to remove or update the DNS record, service.example.com is now pointing to nothing (the cloud host no longer exists). This orphaned entry is a dangling record.
  
  
• Expired or transferred domains: Suppose your organization had a DNS record pointing to an external domain (e.g., a third-party provider’s domain) for a feature or campaign that has since ended. If that external domain expires or you no longer control it, any DNS record still referencing it becomes dangling. The same can happen if you used to own a domain (for a project, etc.) and let it expire while records in another zone still point to it.
  
  
• IP address changes: A records map a name to an IP address. If a server’s IP changes or a cloud instance is deprovisioned, the old IP might be released back into the pool. Any DNS A record still pointing to the old IP could eventually direct to someone else’s server if that IP gets reassigned. The DNS entry is pointing to an IP you do not control – another form of dangling reference.
  
  
• Legacy name servers or mail servers: Sometimes organizations change DNS providers or email providers but forget to clean up all records. An example is an NS record still listing an old name server that is no longer authoritative, or an MX record pointing to an email host that was retired. These unused NS or MX records are dangling entries as well.

In all these cases, the DNS entries in question are no longer tethered to a legitimate, live resource under your control. They remain in your DNS configurations like an unlocked backdoor – and attackers know how to find and exploit these.

To illustrate, imagine a government agency had a subdomain app.agency.gov configured as a CNAME pointing to app.cloudprovider.com for a web service. If the agency later shuts down that service and app.cloudprovider.com is freed up, but the CNAME app.agency.gov is left in place, an attacker could register a new service at app.cloudprovider.com. Suddenly, the attacker now controls app.agency.gov – a subdomain that still belongs to the government agency’s domain. This is not a theoretical scenario; such misconfigurations have occurred in practice, underscoring how dangling DNS records directly lead to subdomain hijacking if left unaddressed.

Why Dangling DNS Records Are Dangerous

Dangling DNS records pose significant security risks. When an attacker manages to take over the resource that a dangling record points to, they gain partial control of your domain’s identity. Here are the key threats and impacts to consider:

• Subdomain Takeover & Impersonation: This is the most notorious risk. A malicious actor who spots a dangling DNS record can register the unassigned service or domain and thereby take over your subdomain. They could then host anything under that subdomain – from a phishing website that looks legitimate because it is on your official domain, to fake login pages or fraudulent services. In the eyes of users (or even automated security filters), content served from a legitimate government or enterprise domain name inherits a level of trust. Attackers abuse that trust to impersonate the organisation, potentially fooling citizens, customers, or employees.
  
  
• Phishing and Malware Delivery: Once an attacker controls a subdomain of yours, they can send convincing phishing emails or deliver malware under the guise of your domain. For example, imagine receiving an email from noreply@service.agency.gov – if service.agency.gov was hijacked via a dangling record, attackers can use it to bypass many spoofing protections. Even robust email security measures like DMARC, which prevent outsiders from spoofing your exact domain, may not stop emails coming from a legitimate (but hijacked) subdomain. This means phishing attacks can appear more authentic, increasing success rates.
  
  
• Data Interception & Session Hijacking: If the dangling subdomain was previously used to host an application, API, or any content that exchanged data with users, an attacker can exploit it to steal information. For instance, if api.example.com was an API endpoint that got forgotten, an attacker controlling it can collect any data or requests sent to that API. Similarly, if a web subdomain is taken over, any cookies or session tokens tied to that subdomain (or wildcard cookies for *.example.com) could be exposed. This can lead to user session hijacking or exposure of sensitive data without the user realizing, because they are interacting with a legitimate domain.
  
  
• Email Hijacking via MX Records: A dangling MX record (mail exchange record) is especially dangerous. MX records direct email for your domain to specific mail servers. If you had an MX pointing to a mail server like mail.provider.com and that domain lapses or is not under your control, an attacker can take over that domain. Consequently, they could start receiving email destined for your organization (for that subdomain or domain). Sensitive emails could be intercepted, and the attacker could even send emails as if from your organization’s address. This kind of breach can lead to significant data loss and confidentiality violations.
  
  
• DNS Zone Manipulation (Dangling NS Records): An often-overlooked scenario is when an old DNS NS record (name server delegation) is dangling. You changed DNS providers but an NS record for the old provider’s server was never removed, or you delegated a subzone to a name server that no longer exists. If an attacker sets up a name server at that old hostname, they can begin answering DNS queries for your domain or subdomain. This is effectively full domain control for the affected portion of your DNS – the attacker can direct any host under that subdomain to wherever they want (e.g., rerouting web traffic, or redirecting validation checks). Even if you have multiple NS records, many resolvers might accept responses from the compromised one, making this a severe threat.
  
  
• Reputation Damage and Trust Erosion: Beyond the direct technical dangers, consider the reputational impact. If an official government or enterprise domain suddenly serves malware or phish, it erodes public trust. Users might lose confidence in the security of your services. For public sector entities, a hijacked subdomain could even become a national security concern if used for disinformation or espionage. The cost to organisational credibility can be high, and recovery (both technical and reputational) is often time-consuming.

Dangling DNS records are not a trivial housekeeping mistake – they are a security vulnerability that adversaries actively seek out. In fact, security researchers have found that this issue is widespread. Studies in recent years uncovered hundreds of thousands of dangling DNS records across the internet, affecting organizations of all sizes. Even well-managed domains in government and education sectors were found to have numerous dangling entries that attackers could exploit. This underlines a key point: no organization is too big or too small to overlook DNS hygiene. The difference between a secure domain and a compromised one often comes down to diligent maintenance.

DNS Hygiene: Preventing and Detecting Dangling Records

The good news is that dangling DNS records are entirely preventable with the right practices – what we can call DNS hygiene. Much like regular hygiene prevents illness, DNS hygiene entails regularly cleaning up and monitoring your DNS configurations to keep them healthy and secure. Here are best practices and measures to help you resolve existing issues and prevent future ones:

• Regular DNS Audits: Schedule periodic reviews of all DNS records in your domains (including secondary domains and subdomains). The goal is to identify any entries pointing to obsolete or unknown targets. Look out for records that return an error or no response (for example, CNAMEs to domains that do not resolve, or A records to unused IPs). Many organizations perform these audits quarterly or biannually, depending on how frequently their infrastructure changes. Modern DNS management tools or external attack surface scanners can help flag broken or suspicious records. By routinely auditing, you can catch a dangling record before an attacker does.
  
  
• Monitor Domain and Service Lifecycles: It is critical to track the lifecycle of the services and domains tied to your DNS. Maintain an inventory of third-party services, cloud resources, and domains used by your organization. When any service is decommissioned or a domain is about to expire, have a process to update or remove related DNS entries immediately. For example, if you shut down an Azure App service, ensure the DNS CNAME pointing to it is removed at the same time. If you discontinue use of a SaaS product that required a DNS entry, delete that entry. And of course, keep an eye on domain expiration dates; never let a domain lapse if your DNS still references it.
  
  
• Integrate DNS Changes into Change Management: Many dangling records persist simply because removing DNS entries was not in the checklist when something changed. To fix this, embed DNS maintenance into your change management process. Whenever a server is migrated, an IP address is changed, or a service is shut down, there should be a mandatory step to verify relevant DNS records. A simple policy could be no cloud resource or service gets decommissioned without confirming its DNS entries are handled. This might involve coordination between application teams and DNS administrators, but it is a necessary step. Having this procedure in place makes it far less likely that an obsolete record will slip through the cracks.
  
  
• Use Cloud Provider Safety Features: Many cloud providers are aware of subdomain takeover issues and offer features to mitigate dangling references. Leverage these when possible. For instance, Azure DNS offers “alias” records that tie a DNS entry to an Azure resource’s lifecycle – if the resource is deleted, the DNS record automatically goes into a safe state (or is easier to spot as unresolved). Azure App Service also provides an optional domain verification TXT record (often called asuid) which, if set, prevents other Azure customers from claiming your DNS name in their services. Similarly, some providers reserve a recently deleted service’s DNS name for a brief period to give you a chance to remove DNS entries or reclaim it in your own account. Take advantage of such features where available, as they add an extra safety net. However, do not rely on them completely – they complement but do not replace good manual hygiene.
  
  
• Automated Scanning and Alerts: Consider using security tools or scripts that automatically scan for dangling DNS records. For example, scripts can query all your subdomains to see if they resolve to expected targets or if they return NX (non-existent domain) responses. Some advanced threat intelligence platforms and DNS security services can continuously monitor your DNS and alert on anomalies like newly broken links. If you are an organization with a large DNS footprint (such as many subdomains across different projects), this kind of automation is invaluable. Even some cloud security solutions (like cloud provider security center or third-party attack surface management tools) will specifically flag potential subdomain takeover risks in your DNS. Early detection allows you to fix an issue before an attacker finds it.
  
  
• Clean-Up of Legacy Records: Over years, especially in large or public sector organizations, DNS zones accumulate a lot of “dusty” records – set up by predecessors or for projects long forgotten. It is understandable that admins can be hesitant to delete something if they are not 100% sure it is unused. However, this leads to the graveyard of dangling records. Cultivate a practice of documenting DNS entries’ purposes and pruning those that are no longer needed. If possible, implement a tagging or description system for DNS records (some DNS management interfaces allow comments) to indicate their owner or function. If a record’s purpose is unclear, investigate it. In many cases you may find it is indeed safe to remove. And if you are truly unsure, at least mark it for future review or test its usage. The fewer stray records you have, the smaller the target for attackers.
  
  
• Improved Accountability and Communication: Often, DNS spans multiple teams (development, operations, networking, etc.). To avoid dangling entries, it helps to assign clear ownership for DNS management and foster communication. For example, if a development team launches a new external service and adds a DNS entry for it, have them inform the DNS admin team and inform them when that service is removed. Some organizations implement a policy that any DNS change must have an associated ticket or change request that documents why it is needed. This creates an audit trail and makes it easier to revisit records later to determine if they are still required. Breaking down silos between teams ensures that DNS records get updated as an integral part of deploying or retiring any service.

By following the above practices, you will significantly reduce the likelihood of leaving dangling DNS records lying around. These measures collectively form a robust DNS hygiene regimen that keeps your domain environment tidy and secure. The goal is to treat DNS management as a critical part of your security posture, not just a set-and-forget networking task.

Conclusion

Dangling DNS records represent a silent yet serious vulnerability in many organizations’ networks. The concept is simple – something was forgotten – but the impact can be far-reaching, from compromised subdomains to full-blown security breaches. The public sector and enterprises alike must recognize that DNS entries are not “configure once and done” assets; they require ongoing maintenance and oversight.

The encouraging news is that with initiative-taking effort, dangling DNS issues are fully preventable. Regular clean-up, vigilant monitoring, and integrating DNS checks into your standard IT processes will close this loophole and strengthen your overall security. In the face of rising cyber threats, there is no easier win than fixing what you already control. It is a straightforward task that can thwart opportunistic attackers who are constantly scanning for that one forgotten subdomain to exploit.

Do not wait until a subdomain takeover or domain hijacking incident forces a lesson. By embracing DNS hygiene best practices now, you protect your organization’s reputation, data, and users from an avoidable attack vector. Remember, security is often about the basics – and keeping your DNS records accurate and up to date is as basic, and critical, as it gets.

Contact us for a free consultation to ensure your DNS infrastructure is secure. Our experts at Excedo Networks can help audit your DNS zones, identify any lingering vulnerabilities like dangling records, and guide you in implementing strong DNS security practices.

Act today to close this gap – a clean DNS house is a safer house in the realm of cybersecurity.