Dangling DNS Vulnerability
A Dangling DNS Vulnerability is a type of security flaw that occurs when a Domain Name System (DNS) record, typically a CNAME (Canonical Name) record, points to a resource or service that has been decommissioned, deleted, or otherwise unclaimed by the organization, but the DNS record itself was never removed.
Detailed Explanation of the Vulnerability
This vulnerability creates a trust paradox that is highly exploitable by attackers, leading to a specific and dangerous type of attack known as a Subdomain Takeover.
The Mechanism of the "Dangle"
Creation of the Alias: An organization creates a CNAME record for a subdomain (e.g.,
staging.company.com) that points to a third-party service, such as a cloud provider, a blogging platform, or a software-as-a-service (SaaS) vendor (e.g.,company-staging.vendor.com).The Inaction: The organization later discontinues use of the third-party service. They delete the account or the specific resource on the vendor's platform but fail to remove the original CNAME record from their own public DNS zone. This is the dangle.
The Exploitation: The attacker discovers this dangling CNAME record. They recognize that the subdomain is still pointing to the vendor but that the specific target resource name (
company-staging) is now available. The attacker simply registers a new account or resource on the vendor's platform using the exact same target name (company-staging.vendor.com).
The Security Consequence: Subdomain Takeover
Once the attacker registers the specific resource name on the vendor's platform, the organization's CNAME record instantly and legitimately directs traffic from their trusted subdomain (staging.company.com) to the attacker's newly created content.
The attacker can then use this compromised, high-authority subdomain for various malicious activities:
Phishing and Impersonation: Hosting a highly credible, secure-looking phishing page under the organization's own domain name to steal credentials.
Malware Distribution: Serving malware or malicious JavaScript from a trusted source, bypassing perimeter defenses.
Cookie Theft: Accessing session cookies that are scoped to the parent domain, leading to full session hijacking.
Brand Damage: Hosting offensive or misleading content that severely damages the organization's reputation.
The vulnerability is particularly potent because the DNS records are public and the attacker's content automatically inherits the trust and security certificates of the legitimate parent domain.
ThreatNG is highly specialized in identifying and mitigating the Dangling DNS Vulnerability by providing comprehensive external visibility and validation, which is exactly what's needed to close the security gap created by the forgotten CNAME record. It systematically replicates the attacker's reconnaissance process to expose and prioritize these high-risk exposures.
ThreatNG's Role in Dangling DNS Vulnerability Mitigation
ThreatNG’s capabilities ensure that the security team is alerted to the "dangle" before an attacker can claim the resource and execute a Subdomain Takeover.
External Discovery
ThreatNG performs purely external unauthenticated discovery to identify all associated subdomains belonging to the organization. This exhaustive search is the first step in finding the forgotten or unmonitored subdomains that might be pointing to decommissioned services.
Example of Discovery Helping Mitigation: ThreatNG performs DNS enumeration to find CNAME records associated with the organization's subdomains. If the organization has a subdomain like oldblog.company.com, ThreatNG will discover that its CNAME record points to an external service like company-blog.ghost.io.
External Assessment (Vulnerability Validation)
ThreatNG’s external assessment capability, specifically the Subdomain Takeover Susceptibility check, directly validates the two conditions that create the Dangling DNS Vulnerability: the CNAME existence and the target's unclaimed status.
The core of the check involves cross-referencing the hostname of the external service against ThreatNG’s comprehensive Vendor List. This list includes numerous services categorized as:
Cloud & Infrastructure: Including PaaS & Serverless vendors like Heroku or Vercel.
Website & Content: Such as Storefront Platforms like Shopify or Content Management like Wordpress.
Customer Engagement: Including Service Desk vendors like Zendesk or Help Scout.
Example of Assessment Helping Mitigation: If the discovered CNAME points to a recognized vendor, ThreatNG performs a specific validation check to determine if the CNAME is currently pointing to a resource that is inactive or unclaimed on that vendor's platform. If the check confirms the "dangling DNS" state (i.e., the subdomain points to a service the organization no longer owns), ThreatNG flags the finding and prioritizes the risk.
Continuous Monitoring and Reporting
Continuous Monitoring ensures that new dangling DNS vulnerabilities are detected immediately as part of changes to the external attack surface, preventing a successful, long-term takeover.
The assessment result on Subdomain Takeover Susceptibility is included in the Reporting and is presented in a Prioritized format (High, Medium, Low, and Informational).
Reporting Example: A report shows a High risk rating for a specific subdomain takeover susceptibility. The report's Knowledgebase provides Reasoning for the identified risk (e.g., CNAME found, resource unclaimed) and clear Recommendations (e.g., remove the CNAME record or reclaim the service) to guide the security team in mitigating the threat.
Investigation Modules and Intelligence Repositories
The Subdomain Intelligence investigation module is the engine behind this entire process, conducting the necessary DNS enumeration and validation.
Subdomain Intelligence: This module is explicitly used to check for Subdomain Takeover Susceptibility by performing the external discovery of associated subdomains, DNS enumeration, and the final validation check against the comprehensive vendor list.
The Intelligence Repositories support the validation process:
The Vendor List used for Subdomain Takeover Susceptibility is kept comprehensive, enabling the system to recognize a vast array of services (from AWS/S3 to Bigcartel to Zendesk) and categorize them, ensuring the check is effective across the modern, complex attack surface.
Cooperation with Complementary Solutions
ThreatNG's ability to precisely validate and locate Dangling DNS issues creates highly actionable data for other security platforms.
DNS Management Solution: ThreatNG identifies and validates a high-risk Dangling DNS Vulnerability on a subdomain like
support.company.com. This specific finding is automatically sent to the organization's DNS Management Solution. The solution then uses the validated ThreatNG finding as a trigger to automate the removal of the specific malicious CNAME record, achieving immediate, accurate mitigation that closes the vulnerability without human error.Security Orchestration, Automation, and Response (SOAR) Platform: Upon discovering and prioritizing a Dangling DNS risk, ThreatNG sends a high-priority alert to a SOAR Platform. The SOAR Platform then uses an automated playbook to notify the relevant asset owner and open a high-priority ticket with a specific action item ("remove CNAME record for X vendor") in the ITSM system, while simultaneously blocking the attacker's ability to successfully claim the service through external filters.
