Subdomain Takeover Prevention
Subdomain takeover prevention involves implementing rigorous, proactive security measures to prevent unauthorized parties from gaining control of a subdomain that was legitimately delegated to an external service but is no longer being properly managed. This attack exploits a concept known as a "dangling DNS record".
A subdomain takeover typically occurs because an organization uses a Canonical Name (CNAME) record in its Domain Name System (DNS) to point a subdomain (e.g., blog.example.com) to a third-party service. If the organization later decommissions or removes the service (e.g., a cloud hosting environment or a web platform) but forgets to delete the corresponding CNAME record, the record remains pointing to an unclaimed or non-existent resource. An attacker can then register an account with that third-party service using the original target name and "claim" the abandoned subdomain, deploying malicious content.
Core Prevention Strategies
Effective prevention requires continuous monitoring and strict lifecycle management for both the DNS record and the external service it points to.
Regular DNS Record Audits: Organizations must routinely review all DNS entries, especially CNAME records, to identify and promptly remove any entries that point to third-party services that are no longer in use or have expired. A clear decommissioning protocol should mandate that the DNS record is removed before or at the same time the external resource is retired.
Continuous Monitoring: Organizations should use automated tools to continuously monitor their entire external attack surface and flag any CNAME records pointing to non-existent or unverified external endpoints. This provides immediate alerts on unauthorized changes or potential vulnerabilities.
Secure Decommissioning Procedures: Clear, standardized processes must be defined for retiring services and subdomains. The preferred order of operations is to remove the DNS record first, then the external service, reversing the creation process.
Inventory and Asset Management: Maintain a comprehensive, real-time inventory that maps every subdomain to its corresponding service provider and current status. This helps track expiration dates and prevents temporary, forgotten subdomains from becoming vulnerable.
Enhanced Access Control: Implement strict access controls, such as multi-factor authentication and domain registrar locking, to prevent unauthorized modifications to DNS settings.
If a takeover is confirmed, the immediate remediation step is to remove the vulnerable DNS record (the CNAME or A record) pointing to the unclaimed external service to break the attacker's control.
ThreatNG is a highly effective platform for Subdomain Takeover Prevention because its design focuses on precisely the external, unauthenticated visibility and continuous validation required to identify and eliminate "dangling DNS" records before an attacker can exploit them.
It moves beyond manual DNS audits by automating the three critical steps of the attack chain: discovery, CNAME analysis, and resource validation.
External Discovery and Continuous Monitoring
ThreatNG ensures that every potential target for a takeover is known and monitored.
External Discovery: The platform’s initial step is to perform purely external unauthenticated discovery to identify all associated subdomains. This process is crucial because organizations often forget about temporary subdomains created for campaigns or testing, which are the primary sources of "dangling" records.
Continuous Monitoring: ThreatNG continuously monitors the external attack surface. This ensures that if a service is decommissioned and the CNAME record is accidentally left behind (creating the dangling DNS state), the newly created vulnerability is detected immediately, not weeks later during a manual audit.
External Assessment (Subdomain Takeover Susceptibility)
ThreatNG has a specialized, multi-step assessment that directly replicates and preempts an attacker's reconnaissance efforts.
CNAME Identification: The assessment begins with DNS enumeration to identify CNAME records pointing to third-party services.
Vendor Cross-Referencing: The core of the check involves cross-referencing the external service's hostname against ThreatNG’s comprehensive Vendor List. This list includes a massive array of services categorized as:
Cloud & Infrastructure: Including storage and CDN like AWS/S3, Cloudfront, Microsoft Azure; and PaaS like Heroku, Vercel.
Website & Content: Including platforms like Shopify and WordPress.
Development & DevOps: Including Version Control like Github.
Dangling DNS Validation: Finally, if a match is found, ThreatNG performs a specific validation check to determine whether the CNAME is currently pointing to an inactive or unclaimed resourceon that vendor's platform. This final step confirms the existence of the "dangling DNS" state and prioritizes the risk.
Security Rating: The result is delivered as a Subdomain Takeover Susceptibility Security Rating (A-F with A being good), providing a precise, quantifiable measure of the risk.
Investigation Modules
ThreatNG’s investigation modules provide the necessary detail to accelerate remediation.
Subdomain Intelligence: This module reports the core findings, detailing which specific subdomains are susceptible to takeover. For example, it might identify a CNAME record on campaign.example.com pointing to a retired Instapage account.
Reconnaissance Hub: This interface can be used to query the findings and quickly pull the Correlation Evidence for the vulnerable subdomain, allowing security teams to validate and prioritize the threat.
Intelligence Repositories
While the intelligence repositories focus on different attack vectors, the comprehensive DarCache Vulnerability repository supports the prevention workflow by providing context on the impact of the takeover. For example, by correlating the vulnerable subdomain with Verified Proof-of-Concept (PoC) Exploits for a related technology, a security team can understand the full scope of what an attacker might deploy on the hijacked domain.
Complementary Solutions
ThreatNG's external validation and prioritization are highly complementary to internal DNS management and provisioning systems.
Working with Identity and Access Management (IAM) Platforms: The discovery of a dangling DNS record indicates a breakdown in asset lifecycle management. ThreatNG can complement an IAM platform by providing an immediate, high-priority alert that is routed to the team responsible for DNS changes. The IAM platform (which manages privileged access to DNS settings) can then enforce a two-person review or a ticketing process for the specific vulnerable DNS zone, ensuring that the decommissioning procedure is followed and the DNS record is removed before the external service is entirely deleted.
Working with IT Service Management (ITSM) Tools (e.g., ServiceNow): When ThreatNG confirms a Subdomain Takeover Susceptibility finding, the Certainty Intelligence and high-priority rating can automatically generate a high-priority ticket in an ITSM system. This ticket would include the exact vulnerable CNAME record, the specific third-party vendor (e.g., Shopify, Heroku) it points to, and the remediation recommendation (delete the CNAME record), directly injecting external, validated risk data into the internal operations workflow. This automates the critical step of removing the vulnerable DNS record that an attacker needs to claim the asset.
