Subdomain Squatting: The Hidden Threat Lurking in Unused DNS Records
Introduction
In the ever-evolving world of cybersecurity, attackers constantly look for vulnerabilities to exploit. While most organizations focus on patching software bugs or securing user credentials, a quieter and equally dangerous threat often flies under the radar—subdomain squatting. This attack vector involves hijacking unused or misconfigured subdomains to distribute malware, steal credentials, or launch sophisticated phishing attacks. In this comprehensive blog, we will explore the nuances of subdomain squatting, how it occurs, its impact, real-world examples, and steps to protect your infrastructure against it.
What is Subdomain Squatting?
Subdomain squatting, also known as subdomain takeover, occurs when an attacker identifies and takes control of a subdomain that points to a non-existent or decommissioned service. This typically happens when a domain’s DNS configuration continues to reference a resource that no longer exists, such as an old cloud service, unhosted page, or decommissioned SaaS platform.
How It Works
- DNS Misconfiguration: A subdomain (e.g., blog.example.com) is configured to point to a third-party service (like GitHub Pages or AWS S3), but the resource at that address has been deleted.
- Scanning: Attackers use automated tools to scan DNS records for such dangling references.
- Claiming the Service: The attacker creates an account or uploads content at the target service using the subdomain’s original identifier.
- Takeover: Since the DNS still points to the third-party service, traffic is routed to the attacker’s controlled content.
Why is Subdomain Squatting Dangerous?
Once an attacker gains control of a subdomain, they inherit its trust and branding. This opens the door to a range of malicious activities:
- Phishing: Launching credential harvesting campaigns that appear to come from a legitimate source.
- Malware Distribution: Hosting malicious files under a trusted domain.
- Brand Damage: Erosion of customer trust in your brand.
- Data Interception: Capturing sensitive information from unsuspecting users.
Real-World Examples of Subdomain Squatting
1. Microsoft Azure
In 2017, security researchers found that many Azure-based subdomains had dangling CNAME records pointing to services that no longer existed, making them vulnerable to takeover.
2. Uber
Uber fell victim to a subdomain takeover in which an attacker used a forgotten subdomain to host a fake login page, stealing user credentials in the process.
3. Starbucks
A subdomain of Starbucks was hijacked to host a cryptojacking script that mined cryptocurrency using the browsers of unsuspecting visitors.
These examples show that even the biggest and most security-aware organizations can be targets of subdomain squatting.
Anatomy of a Subdomain Squatting Attack
To better understand how attackers exploit dangling subdomains, let’s break down the steps involved:
- Reconnaissance: Attackers use tools like Sublist3r, Amass, or DNSDumpster to discover subdomains.
- Validation: They check which subdomains are pointing to services that no longer respond.
- Exploit: If the service allows public sign-ups (e.g., GitHub, Heroku), attackers create a matching resource.
- Payload Deployment: Malicious content is uploaded to the attacker-controlled service.
- Execution: The attacker directs traffic to their malicious content via the hijacked subdomain.
Common Platforms Targeted
Certain cloud providers and platforms are more frequently targeted due to their popularity and configuration practices:
- GitHub Pages
- Amazon S3
- Heroku
- Azure Blob Storage
- Bitbucket
- Shopify
These services often allow users to create content freely, making them ideal for attackers to exploit if DNS records are left unattended.
Detection Techniques
To mitigate the risk of subdomain squatting, it’s essential to proactively identify and remediate vulnerable DNS records:
1. DNS Scanning
Use tools like Subjack, Tko-subs, and Nuclei to automate scanning for dangling subdomains.
2. Manual Auditing
Periodically review DNS entries and compare them with currently active services.
3. Monitoring
Set up alerts for changes in subdomain behavior or sudden spikes in traffic to lesser-used subdomains.
Prevention Best Practices
Preventing subdomain squatting requires a combination of proactive DNS management and internal awareness:
1. DNS Hygiene
- Remove DNS records that are no longer needed.
- Avoid pointing to third-party services unless absolutely necessary.
2. Ownership Tracking
- Keep detailed records of who is responsible for each subdomain.
- Implement lifecycle policies for domains and subdomains.
3. Use of Wildcard Certificates
Avoid wildcard TLS certificates that cover all subdomains. If one subdomain is hijacked, all are compromised.
4. HTTP Security Headers
Use headers like Content-Security-Policy and X-Frame-Options to reduce the impact of compromised subdomains.
5. Automated CI/CD Checks
Integrate subdomain auditing into your deployment pipelines.
Legal and Ethical Considerations
Sometimes researchers discover and report dangling subdomains to responsible companies, but others may exploit them. Subdomain squatting falls into a legal gray area, but using hijacked subdomains for phishing or malware distribution is illegal and unethical.
Organizations should adopt responsible disclosure policies and work with ethical hackers to improve their security posture.
How to Respond to a Takeover
If you discover that a subdomain has been hijacked:
- Remove or Correct the DNS Record: Point it to a valid service or delete it.
- Notify Affected Parties: Inform users, stakeholders, and your IT team.
- Investigate: Determine how the DNS misconfiguration occurred.
- Review Audit Logs: Look for evidence of misuse or credential harvesting.
- Harden Your DNS Practices: Implement lessons learned to prevent future incidents.
Tools and Resources
- Subjack: A popular tool for scanning and identifying vulnerable subdomains.
- Amass: Used for comprehensive asset discovery.
- SecurityTrails: Offers DNS and domain intelligence services.
- Shodan: Useful for discovering exposed infrastructure.
- Cloudsploit: Helps with misconfiguration detection in cloud services.
Case Study: Subdomain Takeover at a Fortune 500 Company
In this section, we’ll walk through a redacted case study of a Fortune 500 company that suffered a subdomain takeover:
- Discovery: A subdomain pointing to an unclaimed GitHub repository.
- Exploitation: Attacker created the same repo and hosted phishing content.
- Detection: Spike in Help Desk tickets alerted the company.
- Resolution: DNS record removed and security protocols revised.
The case underscores the importance of monitoring, ownership, and quick response.
Conclusion
Subdomain squatting is an insidious and often overlooked threat in modern web infrastructure. As organizations increasingly rely on third-party services, the potential for dangling DNS records and associated risks grows. By understanding the threat landscape, implementing preventive measures, and staying vigilant, companies can significantly reduce the risk of subdomain takeovers.
The cost of inaction can be severe—reputational damage, data loss, and financial harm. However, with awareness and proactive security practices, you can keep your domains safe from this silent predator.
