Subdomain hijacking is a highly dangerous and difficult-to-detect attack method in the cybersecurity field. Attackers exploit subdomains no longer used by a target organization, pointing them to malicious servers and launching attacks. This attack method is unique in that it exploits the reputation of legitimate domain names to carry out illegal activities, making it difficult for traditional security devices to effectively identify and block them.

Subdomain hijacking often occurs due to negligence in domain name resolution configuration. When an enterprise points a subdomain to a CNAME record of an external service provider (such as cloud storage, CDN, or SaaS platform), if the contract with the service provider is terminated or the resource is deleted but the DNS record is not updated, an attacker can register the same endpoint on the service provider's platform, thereby taking over control of the subdomain. This security vulnerability allows attackers to exploit the good reputation of the enterprise domain to distribute malicious content or collect user credentials.

Subdomain takeover attacks can take various forms. CNAME takeover is the most common type. When the external resource pointed to by a subdomain is released, the attacker recreates it to hijack the resource. NS record takeover, which occurs when the subdomain's name server records point to DNS servers controlled by the attacker, poses an even more serious risk. Alias ​​takeover involves default domain names provided by cloud service providers. This occurs when an enterprise deactivates the service but fails to remove the corresponding records.

# Use the dig command to check subdomain resolution status
dig CNAME target.subdomain.example.com +short
# Check name server records
dig NS vulnerable.subdomain.example.com +short

A systematic discovery process is the first step in preventing subdomain hijacking. Regularly and automatically scan the DNS records of all subdomains to identify CNAME and NS records pointing to external services. Create an inventory of subdomain assets, noting the business ownership and responsible individuals for each subdomain. Any unused subdomains discovered should be promptly removed or monitored. Use automated tools to continuously verify that the services corresponding to resolution records are valid to prevent dangling references.

python
# Example subdomain monitoring script
import dns.resolver
from datetime import datetime
def check_subdomain_status(domain):
try:
cname = dns.resolver.resolve(domain, 'CNAME')
for answer in cname:
target = answer.target.to_text()
# Verify the existence of the target service
if not verify_service_exists(target):
alert_security_team(domain, target)
except dns.resolver.NoAnswer:
print(f"No CNAME record for {domain}")
except Exception as e:
print(f"Error checking {domain}: {str(e)}")

Lifecycle management forms the core of the defense system. Establish a subdomain application and cancellation process to ensure each subdomain has a clear owner. Automatically trigger subdomain cleanup at the end of a project to prevent unused records from being forgotten. Establish collaboration with cloud and SaaS providers to send notifications upon service termination. Regularly review the necessity of all subdomains and promptly decommission any unneeded entries.

Technical safeguards provide multi-layered defenses. DNS monitoring services can detect suspicious changes to resolution records and identify anomalies promptly. Where possible, use A records instead of CNAME records to directly point to controlled IP addresses. Configure valid TLS certificates for all subdomains to prevent attackers from exploiting invalid certificate warnings to evade detection. Implement strict content security policies to mitigate the potential impact of XSS attacks.

Cloud resource configuration requires special attention. When using cloud storage buckets, web applications, or Function Compute services, ensure resource names are unique to prevent malicious re-registration. When deleting cloud service resources, simultaneously clean up the corresponding DNS records to avoid dangling pointers. Regularly review the mapping between resources and DNS records in your cloud account to ensure consistency.

Automated monitoring systems provide continuous security assurance. Deploy dedicated subdomain monitoring tools to regularly check the resolution status and content of all subdomains. Establish an anomaly alert mechanism to immediately notify the security team when subdomain references change or suspicious content patterns appear. Establish a subdomain reputation evaluation system to identify unusual subdomains that may have been hijacked.

Emergency response plans ensure rapid recovery capabilities. Develop a clear process for handling subdomain hijacking, including confirmation, mitigation, and recovery steps. Prepare a rapid DNS record update plan to immediately discontinue malicious redirection upon confirmation of hijacking. Contact the issuing certificate authority to promptly revoke TLS certificates used maliciously. Maintain complete forensic data for subsequent analysis and legal action.

Defending against subdomain hijacking is an ongoing process that requires a seamless integration of technical controls, process management, and human awareness. By establishing a comprehensive subdomain asset inventory, implementing automated monitoring, and strengthening lifecycle management, enterprises can significantly reduce the risk of subdomain hijacking. As digital businesses increasingly rely on online identities, effective subdomain security management has become an essential component of an enterprise's cybersecurity system.