Subdomain Takeover Scanner

A Subdomain Takeover Scanner is a specialized tool or technique used to identify and detect instances where an organization's subdomain (e.g., blog.example.com, dev.example.com) can be "taken over" by an attacker. This vulnerability arises when a subdomain's DNS record points to an external service or resource that is no longer in use, has been deprovisioned, or is misconfigured, but the DNS entry for that subdomain remains active.

Here's a detailed breakdown:

How Subdomain Takeovers Occur

The core of a subdomain takeover lies in a mismatch between a DNS record and the actual service it points to. Common scenarios include:

1. Dangling DNS Records: An organization might configure a CNAME (Canonical Name) or NS (Name Server) DNS record for a subdomain to point to a third-party service (e.g., a cloud hosting provider, a SaaS application, a content delivery network, a marketing platform). Suppose the organization later deactivates or removes its account with that third-party service, but forgets to remove or update the corresponding DNS record for the subdomain. In that case, the DNS record becomes "dangling."

2. Service Deprovisioning: The third-party service might recycle the resource (e.g., a specific S3 bucket name, a Heroku app name) that the subdomain's DNS record pointed to.

3. Misconfigurations: An organization might accidentally point a subdomain to a non-existent or generic resource on a third-party platform.

In these situations, an attacker can register an account or provision a resource with the same name on the third-party service. Since the organization's DNS record still points to that service and the attacker controls the specified resource, the attacker effectively "takes over" the subdomain.

How a Subdomain Takeover Scanner Works

A subdomain takeover scanner automates the process of identifying these vulnerable subdomains. Its general methodology involves:

1. Subdomain Enumeration: The scanner must first discover as many subdomains associated with a target domain as possible. This can be done through various techniques:

   • Brute-forcing: Guessing common subdomain names (e.g., dev, test, admin, blog, mail).

   • DNS Reconnaissance: Querying DNS records (A, AAAA, CNAME, NS, MX, TXT) for the target domain.

   • Certificate Transparency Logs: Searching public CT logs for subdomain entries.

   • Web Scraping/OSINT: Extracting subdomains from publicly available sources like web pages, GitHub repositories, or search engine results.

   • Third-party databases: Using services that maintain lists of known subdomains.

2. DNS Resolution and Fingerprinting: The scanner performs a DNS lookup for each enumerated subdomain to determine its resolved IP address or CNAME target. It then analyzes the response:

   • If it resolves to a CNAME, the scanner then attempts to identify the underlying service or platform that the CNAME points to (e.g., gh-pages.github.com, s3-website-us-east-1.amazonaws.com, herokudns.com). This often involves looking at the CNAME itself or performing further DNS queries.

   • It also notes if the service returns specific error messages, redirects, or patterns that indicate a non-existent resource (e.g., "NoSuchBucket" for S3, "There's nothing here, yet" for Heroku, or a generic 404 page from a specific hosting provider).

3. Vulnerability Detection (Signature Matching): The core of the scanner's logic lies in identifying these "dangling" or deprovisioned states. The scanner has a database of signatures or patterns associated with various third-party services that indicate a subdomain takeover vulnerability. These signatures include:

   • Specific error messages from cloud providers when a bucket or app doesn't exist.

   • Unique HTTP status codes or response bodies.

   • Specific server headers or other identifying characteristics of deprovisioned services.

   • CNAME records pointing to specific platform domains known to be vulnerable.

4. Proof-of-Concept (Optional but Desirable): Some advanced scanners might even attempt a light Proof-of-Concept (PoC) to confirm exploitability, such as registering the non-existent resource on the identified third-party platform. However, most scanners focus on detection based on the observed DNS and HTTP responses.

Importance in Cybersecurity (especially XTI)

Subdomain takeover scanners are vital for XTI because:

• External Attack Surface Management: They directly contribute to identifying and mapping an organization's external attack surface. A vulnerable subdomain is a direct entry point for attackers.

• Digital Risk Protection: Taken-over subdomains can be used by attackers for various malicious activities:

  • Phishing and Credential Harvesting: An attacker can host a convincing phishing page on a legitimate-looking subdomain (e.g., login.example.com or support.example.com), tricking users into providing credentials.

  • Malware Distribution: The attacker can host malware on the compromised subdomain.

  • Defacement and Reputation Damage: The attacker can put embarrassing or offensive content on the subdomain, damaging the organization's brand reputation.

  • Cross-Site Scripting (XSS) and Session Hijacking: If the main domain uses cookies or scripts that are also valid for subdomains, a compromised subdomain can lead to more severe attacks against the main domain's users.

  • Bypassing Security Controls: Some security controls (e.g., WAFs, email filters) might implicitly trust subdomains of a legitimate organization, making attacks from a taken-over subdomain more effective.

• Proactive Threat Identification: By regularly scanning for and addressing subdomain takeovers, organizations can proactively close a critical attack vector before it can be exploited.

• Compliance: Certain regulatory frameworks may implicitly require organizations to control all their digital assets, including subdomains.

A subdomain takeover scanner is an essential tool for any organization looking to maintain a strong external security posture. It helps identify and mitigate a common yet often overlooked vulnerability that can have severe consequences.

ThreatNG, an all-in-one external attack surface management, digital risk protection, and security ratings solution, significantly assists in addressing subdomain takeovers by offering comprehensive external visibility and actionable insights.

ThreatNG's Contribution to Subdomain Takeover Prevention and Mitigation

1. External Discovery: ThreatNG's ability to perform purely external unauthenticated discovery using no connectors is crucial for identifying subdomains. This means it can find subdomains associated with an organization that might be unknown to internal teams, including those pointing to deprovisioned or misconfigured third-party services. This proactive discovery is the first step in identifying potential subdomain takeover vulnerabilities.

2. External Assessment: ThreatNG provides specific assessment capabilities directly relevant to subdomain takeover susceptibility.

• Subdomain Takeover Susceptibility: ThreatNG directly evaluates a website's subdomain takeover susceptibility. It uses external attack surface and digital risk intelligence that incorporates Domain Intelligence. This intelligence includes a comprehensive analysis of the website's subdomains, DNS records, SSL certificate statuses, and other relevant factors.

  • Example: If ThreatNG discovers a CNAME record for dev.example.com That still points to old-project.heroku.com, but the old-project application on Heroku has been deleted, ThreatNG would flag dev.example.com as highly susceptible to a subdomain takeover. This is because an attacker could register old-project on Heroku and then control dev.example.com.

3. Reporting: ThreatNG offers various reports, including technical and prioritized reports (High, Medium, Low, and Informational). When a subdomain takeover susceptibility is identified, it would be highlighted in these reports. The knowledge base embedded within the reports provides risk levels, reasoning, recommendations, and reference links.

• Example: A technical report would detail the specific CNAME record, the target service, and the reason for the susceptibility. The recommendations would guide the organization on how to remediate the vulnerability, such as removing the dangling DNS record or reclaiming the resource on the third-party service.

4. Continuous Monitoring: ThreatNG monitors external attack surface, digital risk, and security ratings. This ongoing vigilance is critical for subdomain takeover prevention because new subdomains can be created or third-party services can be deprovisioned at any time, leading to new vulnerabilities.

• Example: If an organization launches a new marketing campaign using a temporary subdomain (campaign.example.com) hosted on a third-party platform and later deactivated the campaign without updating the DNS record, continuous monitoring would quickly detect this new dangling DNS entry, preventing a potential takeover.

5. Investigation Modules: ThreatNG's investigation modules provide detailed capabilities for investigating potential subdomain takeover issues.

• Domain Intelligence: This module is foundational, providing DNS Intelligence (Domain Record Analysis, Domain Name Permutations, and Web3 Domains), Email Intelligence, and WHOIS Intelligence. Crucially, Subdomain Intelligence is a part of this module.

  • Subdomain Intelligence: This provides a comprehensive analysis of subdomains, including HTTP Responses, Header Analysis, Server Headers, Cloud Hosting, Website Builders, E-commerce Platforms, Content Management Systems, and content identification. It also includes explicit Subdomain Takeover Susceptibility as a direct capability.

  • Example: An analyst could use the Subdomain Intelligence module to review all known subdomains, their resolved CNAMEs, and the HTTP responses received from the target services. If a subdomain like careers.example.com points to examplecorp.breezy.hr via CNAME, and Breezy HR returns a specific "account not found" error, the module would flag it as vulnerable. The analyst could verify this using the module's resolved technologies and content identification details.

6. Intelligence Repositories (DarCache): While not directly for subdomain takeover detection, the DarCache repositories provide valuable contextual intelligence that enhances the overall XTI posture and understanding of why subdomain takeovers are critical.

• DarCache Vulnerability (NVD, EPSS, KEV, PoC Exploits): This repository helps organizations understand the broader threat landscape and the potential impact of any vulnerability, including a subdomain takeover. For instance, knowing that specific platforms commonly used for subdomain takeovers have associated CVEs (from NVD) or are frequently exploited (from KEV) can raise the urgency for remediation. The DarCache eXploit, with direct links to Proof-of-Concept exploits, helps security teams understand how a subdomain takeover could be weaponized.

Examples of ThreatNG Helping with Subdomain Takeovers

• Uncovering Legacy Assets: An organization recently migrated its old blog from a third-party platform to an internal server. ThreatNG's external discovery and continuous monitoring identify that oldblog.example.com it still has a CNAME record pointing to the old platform. ThreatNG's Subdomain Takeover Susceptibility assessment flags this as vulnerable because the old platform account has been deprovisioned. The organization receives a report detailing this risk. Removing the dangling DNS record is advised, preventing an attacker from claiming the old platform's resource and taking over the subdomain.

• Detecting Misconfigured DNS: During a routine scan, ThreatNG identifies support.example.com pointing to a generic cloud storage URL via CNAME. The content identification in the Subdomain Intelligence module shows a "bucket not found" error from the cloud provider, indicating a misconfiguration. ThreatNG flags this as a subdomain takeover risk, allowing the organization to correct the DNS entry or provision the correct bucket, thus securing the subdomain before it can be used for phishing.

Synergies with Complementary Solutions

ThreatNG can work effectively with other cybersecurity solutions to create a more robust defense against subdomain takeovers.

• DNS Management Systems: ThreatNG's identifying vulnerable subdomains can trigger automated remediation actions within an organization's DNS management system. For example, if ThreatNG flags a dangling CNAME record, the DNS management system could be configured to automatically remove or update that record based on a predefined policy, significantly speeding up the remediation process.

• Security Information and Event Management (SIEM) / Security Orchestration, Automation, and Response (SOAR) Platforms: ThreatNG's alerts about subdomain takeover vulnerabilities can be ingested by a SIEM for centralized logging and correlation with other security events. A SOAR platform could then automate the workflow for responding to such alerts, perhaps by automatically creating a ticket for the network team, triggering a re-scan of the affected subdomain, and notifying relevant stakeholders.

• Threat Intelligence Platforms (TIPs): While ThreatNG provides its intelligence repositories, a TIP could aggregate ThreatNG's findings on vulnerable subdomains and integrate them with broader threat intelligence feeds about attacker tactics for exploiting subdomain takeovers. This could provide even richer context and help prioritize remediation based on real-time threat actor activity.

• Attack Surface Management (ASM) platforms (for internal view): While ThreatNG focuses on the external attack surface, an internal ASM platform could complement its findings by identifying internal systems that might be linked to vulnerable external subdomains, helping to understand the full blast radius if a takeover were to occur.