Subdomain Takeover: The Forgotten DNS Records Hijacking Your Brand 🌐

Introduction

In the rapidly evolving landscape of cybersecurity, one vulnerability continues to plague organizations of all sizes: subdomain takeover. While security teams focus on patching software vulnerabilities and defending against sophisticated attacks, abandoned cloud resources and forgotten DNS records create invisible backdoors that attackers can exploit in mere hours. Cybercriminals can exploit an unused subdomain to host phishing sites, spread malware, and capture sensitive user data in hours, transforming your trusted domain into a weapon against your own customers.

This comprehensive guide explores the mechanics of subdomain takeover, reveals how attackers discover and exploit these vulnerabilities, and provides actionable strategies to protect your digital infrastructure. Whether you’re a security professional, developer, or business owner, understanding this threat is crucial for maintaining your brand’s integrity and protecting your users.

What is Subdomain Takeover?

Subdomain takeover vulnerabilities occur when one of the company’s subdomains points to a third-party service that no longer exists, allowing an attacker to claim that service and control what appears on the subdomain. This seemingly simple misconfiguration creates a dangerous scenario where attackers can serve malicious content from domains that users inherently trust.

The Anatomy of a Vulnerable Subdomain

The vulnerability typically emerges through a predictable sequence of events:

Service Provisioning: Your development team creates a subdomain (e.g., marketing.yourdomain.com) and points it to a third-party service like AWS S3, Azure, GitHub Pages, or Heroku.
DNS Configuration: A DNS record (usually a CNAME) is created to route traffic from your subdomain to the external service.
Service Decommissioning: The project ends, the cloud resource is deleted, but the DNS record remains in place.
The Vulnerability Window: Such DNS records are also known as “dangling DNS” entries, creating an opportunity for attackers to claim the same resource name and control your subdomain’s content.

Why This Matters to Your Business

Subdomain takeovers pose significant security risks: Phishing attacks – attackers can host malicious content on the subdomain, leveraging the trust associated with the main domain. Data theft – sensitive information can be stolen if users are tricked into interacting with the compromised subdomain. The implications extend beyond technical security concerns to encompass brand reputation, customer trust, and regulatory compliance.

Real-World Impact: Recent Examples

The threat of subdomain takeover isn’t theoretical. Organizations worldwide continue to fall victim to these attacks, with devastating consequences.

The SubdoMailing Campaign

Recently, a massive ad fraud campaign named “SubdoMailing” used over 8,000 legitimate domains to send fraudulent emails at scale. By exploiting dangling DNS records, attackers leveraged the reputation of trusted domains to bypass email security filters, demonstrating how subdomain takeovers can enable sophisticated, large-scale attacks.

Supply Chain Implications

In this investigation from October 2024 to January 2025, the researchers sought deleted AWS S3 buckets with existing references pointing to them, specifically seeking buckets that may have been used within pipeline management, application development and deployment. This research highlighted how subdomain takeovers can morph into supply chain attacks, potentially compromising development pipelines and software distribution mechanisms.

High-Profile Incidents

Government websites, Fortune 500 companies, and popular platforms have all experienced subdomain takeovers. You may have seen the news about the defacement of the Trump administration website as an example. These incidents underscore that no organization is immune to this vulnerability, regardless of size or security maturity.

Common Vulnerable Services and Platforms

Understanding which services are commonly exploited helps prioritize your security efforts. Here are the platforms most frequently targeted:

Cloud Storage Services

Amazon S3: If the developer deletes the bucket but forgets to remove the corresponding DNS record, he’ll have inadvertently introduced a new subdomain takeover vulnerability. An attacker can then create a new AWS S3 bucket with an identical name through the AWS console and take control of all content.

Azure Blob Storage: Similar to S3, when storage accounts are deleted but DNS records persist, attackers can recreate accounts with matching names.

Platform-as-a-Service (PaaS)

Heroku: Deleted apps leave behind CNAME records pointing to herokuapp.com domains that can be reclaimed.
GitHub Pages: Repositories deleted without removing corresponding DNS entries create takeover opportunities.
Shopify: E-commerce projects migrated or abandoned often leave vulnerable DNS configurations.
Fastly and CloudFront: CDN configurations deleted while DNS records remain active.

Content Management and Marketing Platforms

WordPress.com: Hosted blog subdomains that have been deactivated.
HubSpot: Marketing automation platforms with abandoned landing pages.
Zendesk: Customer support subdomains pointing to deactivated help centers.

Step-by-Step Guide: Finding Subdomain Takeover Vulnerabilities

Security professionals and ethical hackers use systematic approaches to identify these vulnerabilities. Understanding the process helps both defenders and authorized testers.

Step 1: Subdomain Enumeration

The first phase involves discovering all subdomains associated with a target domain:

Passive Methods: - Certificate Transparency Logs: Search crt.sh for all certificates issued for the target domain - DNS Aggregators: Use services like SecurityTrails, DNSDumpster, or VirusTotal - Search Engine Reconnaissance: Utilize Google dorks (e.g., site:*.example.com)

Active Methods: - DNS Brute Forcing: Use tools like Sublist3r, Amass, or Subfinder to test common subdomain names - Zone Transfer Attempts: Check if DNS servers allow zone transfers (misconfiguration)

Step 2: DNS Record Analysis

Once subdomains are identified, analyze their DNS records:

# Check CNAME records
dig marketing.example.com CNAME

# Look for NS records
dig dev.example.com NS

# Examine A records
dig blog.example.com A

Look for responses indicating external services, particularly error messages like “NXDOMAIN” or records pointing to third-party platforms.

Step 3: Vulnerability Identification

If the process of provisioning or deprovisioning (removing) a virtual host is not handled properly, there can be an opportunity for an attacker to take over a subdomain. Key indicators of vulnerability include:

CNAME pointing to non-existent resources: The DNS record exists, but the target resource doesn’t
Error messages: “404 Not Found,” “NoSuchBucket,” “There isn’t a GitHub Pages site here”
Unclaimed usernames: The service allows you to claim the exact name referenced in the DNS record

Step 4: Verification Testing

Before reporting a vulnerability, verify it’s exploitable:

Claim the resource: On the target platform, create an account/resource matching the DNS record’s target
Deploy test content: Place a unique, harmless identifier (e.g., “Proof of Concept by [Your Name]”)
Access via subdomain: Visit the vulnerable subdomain to confirm your content appears
Document thoroughly: Screenshot the process, DNS records, and successful takeover

Tools for Automated Detection

Several specialized tools streamline subdomain takeover detection:

SubOver: Python-based tool checking for takeover vulnerabilities across multiple services
subjack: Go-based fingerprinting tool with extensive service signatures
Nuclei: Extensible vulnerability scanner with subdomain takeover templates
Can I Take Over XYZ: Comprehensive database of takeover-vulnerable services

The Exploitation Process (Ethical Perspective)

Understanding how attackers exploit subdomain takeovers helps security teams think like adversaries and strengthen defenses.

Reconnaissance Phase

Attackers begin with broad reconnaissance, identifying organizations with large digital footprints and numerous subdomains. They target: - Companies undergoing mergers or acquisitions (increased likelihood of abandoned resources) - Organizations with frequent marketing campaigns (temporary landing pages) - Tech companies with numerous development projects

Claiming the Resource

Once a vulnerable subdomain is identified, the exploitation is often straightforward:

Create an account on the identified service (AWS, Azure, GitHub, etc.)
Provision a resource with the exact name referenced in the DNS record
Deploy malicious content designed to exploit user trust

Malicious Activities

With control established, attackers can:

Phishing Operations: Adversaries don’t need to force their way in when they can slip through an organization’s overlooked assets. They create convincing login pages on trusted subdomains to harvest credentials.

Malware Distribution: Serve exploit kits or malicious downloads from domains that security software trusts.

Data Harvesting: Collect cookies, session tokens, and user-submitted information intended for legitimate services.

Reputation Damage: Deface subdomains with embarrassing or inappropriate content to harm brand reputation.

SEO Manipulation: Create spam content on high-authority subdomains to manipulate search rankings.

Comprehensive Defense Strategy

Protecting against subdomain takeover requires a multi-layered approach combining technical controls, process improvements, and organizational awareness.

Immediate Actions

Conduct a Subdomain Audit: Enumerate all your organization’s subdomains and document their purpose and ownership.

Inventory DNS Records: Create a complete inventory of DNS records, particularly CNAMEs pointing to external services.

Identify Orphaned Records: Subdomains are vulnerable when DNS entries point to decommissioned services or expired resources. For example, if your team retires a project hosted on Amazon S3 but the DNS record still points to a non-existent bucket, attackers can claim and exploit the bucket name.

Remove Dangling References: Delete DNS records that no longer serve active resources.

Process and Policy Improvements

Establish Decommissioning Procedures: Create formal processes ensuring DNS records are removed before cloud resources are deleted. Make DNS cleanup a mandatory step in project termination checklists.

Implement Change Management: Require approval and documentation for all DNS changes, with periodic reviews to validate record necessity.

Assign Ownership: Designate clear ownership for every subdomain and DNS record, ensuring accountability for maintenance.

Quarterly Reviews: Schedule regular audits of DNS records and cloud resources to identify and address misconfigurations.

Technical Controls

DNS Monitoring: Implement continuous monitoring solutions that alert on DNS changes and identify dangling records.

Automated Scanning: Deploy tools like SubOver or Nuclei on regular schedules to proactively detect vulnerabilities.

Cloud Resource Tagging: Tag all cloud resources with project information and DNS associations for easier tracking.

Prevent Resource Reuse: Some cloud providers offer options to permanently delete resources, preventing name reuse.

Wildcard Records Caution: While wildcard DNS records (*) can mitigate some takeover risks, they introduce other security concerns and should be carefully evaluated.

Cloud Provider Protections

Major cloud providers have implemented protections, but understanding their limitations is crucial:

Azure: CNAME records are especially vulnerable to this threat, but Azure has implemented domain verification requirements for certain services, requiring ownership proof before custom domains can be added.

AWS: Route 53 offers alias records that automatically remove themselves when target resources are deleted, though CNAME records remain vulnerable.

Google Cloud: Domain verification processes for many services reduce but don’t eliminate takeover risks.

Security Tools and Monitoring

Integrate subdomain takeover detection into your security workflow:

Continuous Monitoring Platforms: Services like SecurityTrails, DNSdumpster, or Detectify continuously monitor for DNS changes and vulnerabilities.

Bug Bounty Programs: Incentivize external security researchers to identify and report subdomain takeover vulnerabilities through platforms like HackerOne or Bugcrowd. HackerOne’s Hacktivity feed — a curated feed of publicly-disclosed reports — has seen its fair share of subdomain takeover reports.

SIEM Integration: Configure your Security Information and Event Management system to alert on DNS changes and unusual subdomain activity.

Responding to a Subdomain Takeover

If you discover an active subdomain takeover, respond swiftly:

Immediate Response

Verify the Takeover: Confirm the subdomain is indeed compromised and document the malicious content.
Remove DNS Records: Immediately delete the DNS record pointing to the compromised resource. This stops traffic to the attacker’s content.
Notify Stakeholders: Inform security teams, legal counsel, and affected business units.
Assess Impact: Determine what content was served, how long the takeover existed, and potential victim count.

Investigation Phase

Review Access Logs: If available, analyze DNS query logs to understand the scope of exposure.
Check for Data Compromise: Investigate whether user data was collected or credentials phished.
Forensic Analysis: Document the attack methodology for future prevention.

Post-Incident Actions

Implement Preventive Controls: Address the root cause and apply lessons learned across your infrastructure.
User Notification: If user data was compromised, comply with breach notification requirements.
Regulatory Reporting: Report incidents to relevant authorities as required by applicable regulations.

Best Practices for Long-Term Protection

Sustainable security requires embedding subdomain takeover prevention into organizational culture:

Development Team Education

Train developers on subdomain takeover risks, emphasizing: - The importance of DNS hygiene - Proper decommissioning procedures - Secure cloud resource management

Infrastructure as Code (IaC)

Manage DNS records through version-controlled infrastructure-as-code: - Track all DNS changes in Git repositories - Implement automated validation to detect dangling records - Use Terraform, CloudFormation, or similar tools for DNS management

Security Awareness

Include subdomain takeover in security awareness programs: - Explain the business impact to non-technical stakeholders - Demonstrate how seemingly minor oversights create major vulnerabilities - Foster a culture where security is everyone’s responsibility

Vendor Management

When working with third-party services: - Document which services have DNS records pointing to them - Review vendor contracts for clauses addressing resource decommissioning - Establish communication channels for notifying vendors of security concerns

Conclusion

Subdomain takeover represents a perfect storm of forgotten infrastructure, rapid cloud adoption, and inadequate decommissioning processes. Adversaries don’t need to force their way in when they can slip through an organization’s overlooked assets. Subdomain takeovers are a prime example of how attackers exploit seemingly minor misconfigurations to achieve significant impact.

The good news is that subdomain takeover is entirely preventable through diligent DNS management, proper decommissioning procedures, and continuous monitoring. By implementing the strategies outlined in this guide—from immediate audits to long-term process improvements—organizations can eliminate this vulnerability from their attack surface.

Remember that security is not a destination but a journey. Regular audits, automated monitoring, and organizational awareness transform subdomain takeover from an invisible threat into a managed risk. Take action today to inventory your subdomains, clean up dangling DNS records, and implement processes that prevent future vulnerabilities.

Your brand’s reputation and your users’ trust depend on the invisible infrastructure of DNS records. Don’t let forgotten configurations become the backdoor that undermines your security investment. Start your subdomain audit today, because attackers are already searching for your dangling DNS entries.

About the Author: This article was researched and written with current cybersecurity best practices as of October 2025, drawing from industry-leading sources including Microsoft Azure Security, CrowdStrike, OWASP, HackerOne, and recent threat intelligence reports on subdomain takeover campaigns.