Subdomain Takeover Validation
Subdomain Takeover Validation is a critical, proactive cybersecurity process used to verify the existence, exploitability, and potential impact of a subdomain takeover vulnerability affecting an organization’s digital assets. It involves attempting to claim an expired or dangling subdomain safely and methodically to confirm that an attacker could successfully hijack it.
Understanding the Subdomain Takeover Risk
A subdomain takeover vulnerability occurs when a domain name registrar record (e.g., a CNAME) for a subdomain (e.g., test.company.com) points to an external service (e.g., an AWS S3 bucket, GitHub Pages, or anAzure service) that has been decommissioned, deleted, or otherwise abandoned by the organization.
The crucial point is that the DNS record remains active and points to the external service's unique address, but the external service instance itself is no longer owned or registered.
The Validation Process
Validation moves the risk from theoretical to confirmed and involves several key stages:
1. Discovery and Identification
This initial stage focuses on finding candidate subdomains that might be susceptible.
DNS Enumeration: Organizations scan their entire domain space for CNAME records that point to external hosting providers.
Status Check: Tools check the HTTP response code from the external target service. A standard indicator of a potential takeover is a specific error message from the cloud provider (e.g., "Bucket not found," "No such app") rather than a generic HTTP 404 or 503 error. This message confirms the external resource is dangling.
2. Proof of Concept (PoC) Confirmation
This is the core of the validation process, carried out in a secure, non-malicious manner.
Safe Registration: The security team, acting as an ethical attacker, attempts to register the abandoned external resource using the same service name referenced in the dangling CNAME record. For example, if test.company.com points to a missing S3 bucket named company-test-bucket, the team tries to create a new S3 bucket with that exact name.
Verification Payload: If the registration is successful, the team uploads a benign, unique HTML file or text string (the "payload") to the newly claimed resource.
External Access Check: The final step is visiting the original subdomain (test.company.com) in a browser. If the browser displays the security team's unique payload instead of the cloud provider's error message, the subdomain takeover is validated and confirmed as exploitable.
3. Impact Assessment
After validation, the team quantifies the potential damage. Since the attacker controls the subdomain, they can use it for:
Session Hijacking: Deploying malicious code to steal cookies or session tokens from users visiting the subdomain.
Phishing and Brand Abuse: Hosting a credible phishing page to steal credentials, leveraging the trust associated with the organization's valid domain name.
Malware Distribution: Using the legitimate domain to distribute malicious files, bypassing some security controls that trust the domain.
The ultimate goal of Subdomain Takeover Validation is to generate the evidence needed to urgently remove the dangling DNS record, thereby mitigating the risk.
ThreatNG is exceptionally well-suited to handle Subdomain Takeover Validation because its design provides the necessary external discovery, continuous monitoring, and assessment capabilities to identify, confirm, and prioritize these vulnerabilities at scale, without requiring manual, high-risk investigation. It automates the entire validation lifecycle.
ThreatNG's Process for Subdomain Takeover Validation
1. External Discovery and Continuous Monitoring (Identification Stage)
These modules automatically perform the initial, labor-intensive work of finding dangling subdomains across the organization's entire digital presence.
External Discovery: ThreatNG autonomously and continuously maps all of an organization's domain assets, including thousands of subdomains that may have been forgotten. It identifies CNAME records pointing to external providers (e.g., Azure, GitHub, Amazon). This automated mapping is the critical first step in finding all potential takeover targets.
Continuous Monitoring: ThreatNG constantly observes the status of these external services. It tracks the specific error messages (e.g., "service not found") returned by the cloud providers when the external asset is accessed. ThreatNG identifies the pattern of a "dangling" CNAME—a record that exists but points to a service that doesn't. This effectively automates the high-level identification of takeover candidates, which would otherwise be a tedious manual process.
2. External Assessment and Intelligence Repositories (Validation Stage)
These modules automate the confirmation of exploitability and determine the severity, effectively replacing the manual Proof of Concept (PoC) step with an intelligent, automated verification.
External Assessment
This feature validates the existence and ease of exploitation for the dangling subdomain. ThreatNG's assessment confirms the technical viability of the takeover.
Detailed Examples of External Assessment:
Exploitability Check: ThreatNG finds a subdomain, careers.company.com, which points via CNAME to an Amazon CloudFront distribution that is no longer active. The assessment module not only flags the dangling CNAME but also attempts to determine whether the naming convention of the abandoned service is generic enough for an attacker to register easily. This provides the exploitability context.
Target Service Analysis: For a subdomain pointing to an Azure service, the assessment identifies the specific Azure error code indicating a potential takeover. It then provides the attributable risk context by confirming that the subdomain, beta.company.com, is known to handle user login attempts. The assessment automatically assigns a maximum risk score because a successful takeover would allow an attacker to collect user credentials via a highly trusted domain.
Intelligence Repositories
These repositories enhance the validation by providing up-to-date knowledge of known takeover techniques and associated risks.
ThreatNG's intelligence includes a constantly updated list of unique signatures and error messages associated with vulnerable takeover targets across central cloud and hosting providers. This knowledge enables the platform to perform more precise, definitive automated validation of the takeover risk, ensuring the finding is a true positive.
3. Investigation Modules and Reporting
These modules provide the necessary context for rapid mitigation once the takeover risk is validated.
Investigation Modules
These modules consolidate all the evidence required for a security team to execute the mitigation confidently.
Detailed Examples of Investigation Modules in Use:
Comprehensive Evidence: An analyst sees a validated subdomain takeover alert. The Investigation Module instantly displays the full context: (1) The specific dangling CNAME record (dangling.company.com), (2) The specific cloud provider and the exact error message that confirms the dangling state, and (3) The risk rating, which is justified by the fact the subdomain is currently linked to the organization’s primary email authentication system (asset criticality context). This unified view bypasses the need for the analyst to manually perform DNS lookups and web checks, dramatically reducing time-to-remediation.
Ownership Tracing: The module allows the security analyst to query the asset history, quickly identifying the original team or individual responsible for provisioning the external service. This provides the necessary internal context to coordinate the deletion of the dangling DNS record with the correct internal owner.
Reporting
ThreatNG’s Reporting module translates the validated takeover risk into an urgent, unambiguous call to action for operations teams. It clearly states: "Subdomain takeover risk confirmed on [subdomain name]," accompanied by the technical evidence and the required remediation step (delete the DNS record).
Examples of ThreatNG Helping:
Prevented Hijacking: ThreatNG identifies a critical subdomain takeover vulnerability on archive.company.com. The high risk score and clear validation evidence allow the operations team to delete the dangling CNAME record within minutes of the alert, preventing an attacker from claiming the asset and using it in a session-hijacking campaign.
Mitigating Brand Abuse: A validated subdomain takeover is found on a forgotten asset (oldblog.company.com). ThreatNG's report is used to justify the immediate removal of the DNS record, preventing an adversary from using the subdomain to host a highly credible phishing site targeting customers.
4. Working with Complementary Solutions
ThreatNG collaborates with other systems to ensure seamless, automated mitigation of takeover risks.
Cooperation with DNS Management Systems: ThreatNG can integrate with the organization’s authoritative DNS management system. For a highly validated takeover risk, ThreatNG can be configured to automatically trigger an API call to the DNS management system to delete the specific, dangling CNAME record. This cooperation turns the validation finding into an immediate, automated fix.
Example: Upon validation, ThreatNG sends an instruction to the DNS platform to delete the CNAME record for dev-site.company.com, eliminating the risk instantly without human intervention.
Cooperation with Asset Inventory/CMDBs: ThreatNG feeds its discovery of all subdomains and the specific vulnerability findings into the organization’s asset inventory. This cooperation ensures that the CMDB is updated with the correct status and risk score for that particular asset, improving data fidelity across all internal systems.
Example: The CMDB is updated with the status "VULNERABLE: Subdomain Takeover Risk." Internal audit tools can then use this tag to confirm that the asset has been properly decommissioned or repaired, thereby closing the risk loop.
