Subdomain Takeover Susceptibility
Subdomain Takeover Susceptibility in cybersecurity refers to the risk that a threat actor can seize control of a seemingly legitimate subdomain belonging to an organization, such as blog.example.com, careers.example.com, or dev.example.com.
The Core Mechanism
This vulnerability arises when a subdomain's Canonical Name (CNAME) record points to an external third-party service or platform (such as a hosting provider, content delivery network, software-as-a-service application, or project management tool) that the organization no longer uses or has decommissioned.
The key factors are:
Dangling DNS Record: The Domain Name System (DNS) record for the subdomain is never updated. The CNAME record still points to the external vendor's hostname (e.g., example.vercelservice.com).
Inactive External Resource: The organization has deleted the corresponding account, page, or service on the external vendor's platform. The vendor's hostname is now "unclaimed" or "inactive."
The Attack Scenario
A threat actor can exploit this vulnerability by registering an account with an external third-party service and then claiming the target organization's inactive hostname, which the target organization's CNAME record still points to. Once the threat actor claims the hostname, they effectively control the subdomain.
The attacker can then host arbitrary malicious content on the victim's legitimate subdomain, which may include:
Phishing Pages: Creating a compelling login page to steal user credentials.
Malware Distribution: Hosting malicious files or links that appear to come from a trusted source.
Defacement and Brand Damage: Posting offensive or inaccurate content to damage the organization's reputation.
Session Hijacking: Exploiting the subdomain's inherited security context to steal cookies or compromise user sessions on the main domain.
Impact and Mitigation
The impact of a successful subdomain takeover is significant because the subdomain inherits the parent domain's trust and reputation. Since the malicious content is served from a legitimate domain, security filters and end users are less likely to flag it as suspicious, thereby enabling successful social engineering attacks.
Mitigation primarily involves a careful DNS cleanup process:
Continuous Monitoring: Regularly auditing and checking all subdomains for dangling DNS pointers.
Decommissioning Protocol: When retiring a third-party service, the CNAME record for the associated subdomain must be removed or updated before or immediately after deleting the external resource or account.
Wildcard DNS: Using wildcard DNS records sparingly, as they can inadvertently increase the attack surface.
The ThreatNG platform provides a comprehensive solution for managing the risk of Subdomain Takeover Susceptibility by integrating its core capabilities first to detect potential vulnerabilities and then provide actionable intelligence for mitigation.
ThreatNG's Role in Subdomain Takeover Susceptibility
External Discovery
ThreatNG starts by performing purely external unauthenticated discovery to identify all associated subdomains of an organization. This initial step provides the necessary inventory for the subsequent analysis. Within the Domain Intelligence investigation module, the platform uses its DNS Intelligence module to identify CNAME records pointing to third-party services.
External Assessment and Security Ratings
The discovered subdomains are assessed for Subdomain Takeover Susceptibility, resulting in a Security Rating ranging from A (good) to F (bad).
Detailed Examples of Assessed Third-Party Services: The core of the assessment involves cross-referencing the external service's hostname against a comprehensive Vendor List. This list is broken down into detailed categories that indicate whether the pointing CNAME is inactive or unclaimed on that vendor’s platform, thereby confirming the "dangling DNS" state and prioritizing the risk.
Cloud & Infrastructure: Includes Storage & CDN, such as AWS/S3 and CloudFront; PaaS & Serverless, such as Heroku and Vercel; and CDN/Proxy, such as Fastly.
Website & Content: Includes Content Management like WordPress and Pantheon, and Storefront Platforms like Shopify and Bigcartel.
Customer Engagement: Includes Service Desk solutions like Zendesk and Freshdesk.
Development & DevOps: Includes Version Control platforms like GitHub and Bitbucket.
Investigation Modules
The specialized process is run within the Subdomain Intelligence investigation module.
Detailed Examples of the Investigation Process:
External Discovery is performed to identify all associated subdomains.
DNS enumeration is used to find CNAME records pointing to external third-party services.
A cross-referencing check is performed against the comprehensive Vendor List (including examples like AWS/S3, Heroku, Shopify, and Zendesk).
A specific validation check is performed to determine whether the CNAME record points to an inactive or unclaimed resource on the vendor's platform, confirming the "dangling DNS" state and prioritizing the risk.
Continuous Monitoring and Reporting
ThreatNG provides Continuous Monitoring of the external attack surface and digital risk. This ensures that new dangling DNS records or changes in vendor platform status are constantly tracked. The findings are compiled into various report formats.
Reporting Examples: The specific subdomain takeover risk, its priority (High, Medium, Low), and the resulting security rating (A-F) are included in Prioritized and Security Ratings reports. This allows the security team to understand the risk and justify remediation efforts.
Intelligence Repositories
The DarCache Ransomware repository tracks over 70 ransomware gangs. While not directly identifying a dangling DNS, the overall platform's intelligence context is crucial.
Detailed Example of Intelligence Support: The comprehensive Vendor List used in the assessment is an internal intelligence repository that ThreatNG continuously updates, enabling it to accurately identify unclaimed services across a wide range of third-party platforms.
Cooperation with Complementary Solutions
ThreatNG's ability to provide Contextual Risk Intelligence through its Context Engine™ and Legal-Grade Attribution makes its subdomain-takeover findings highly valuable when used alongside other security tools.
Example of ThreatNG Helping: ThreatNG discovers that a subdomain, careers.example.com, has a CNAME record pointing to an unclaimed Greenhouse service. The platform provides the risk rating (e.g., F) and the specific recommendation to remove the CNAME record. The finding is also automatically correlated with specific MITRE ATT&CK techniques, such as Initial Access, enabling security leaders to prioritize threats based on their likelihood of exploitation.
Example of ThreatNG and Complementary Solutions Cooperation:
ThreatNG detects the vulnerable careers.example.com subdomain, confirms the dangling DNS state, and provides Legal-Grade Attribution that eliminates guesswork.
A complementary Security Orchestration, Automation, and Response (SOAR) platform could use this high-certainty finding from ThreatNG to trigger a remediation workflow automatically:
It could alert the DNS administrator via email (using the Admin email discovered by NHI Email Exposure ) with the specific CNAME record to delete.
It could then automatically generate a high-priority ticket in the organization's existing IT Service Management (ITSM) solution (such as ServiceNow, if identified by SaaSqwatch) with all the context and risk rating, accelerating the fix.
