Detecting Unknown Subdomains with Certificate Transparency — A Practical Introduction Using Cert Spotter

A practical guide to detecting new subdomains and SSL/TLS certificates issued under a root domain you own, from the outside. It walks through Certificate Transparency (CT) log fundamentals, subdomain discovery and continuous monitoring with the Cert Spotter API and crt.sh, the blind spots created by wildcard certificates and wildcard DNS, and a recommended layered setup that combines CT with Passive DNS.