Subdomain (Subdomain)
Why Subdomain Matters
Subdomains are commonly used to organize different services: www for the website, api for the backend, mail for email, staging for test environments. Each subdomain can point to different servers or services, providing architectural flexibility.
From a security perspective, every subdomain increases your attack surface. Each subdomain needs its own certificate coverage (wildcard or SAN), may run different software with different vulnerabilities, and can be a target for subdomain takeover if it points to a decommissioned service. A forgotten staging.example.com running outdated software is a common entry point for attackers.
Subdomain enumeration is a standard reconnaissance technique. Attackers use tools to discover all subdomains of a target, looking for forgotten or poorly secured services. Maintaining an inventory of all active subdomains and regularly auditing them is an important security practice. Remove DNS records for subdomains you no longer use.
How to Check
A DNS health checker shows the subdomains associated with your domain. Review each one to verify it is still needed, points to an active service, and is properly secured. Check for dangling CNAME records that could enable subdomain takeover.