What this tool checks
Every publicly-trusted SSL certificate issued anywhere is permanently logged in Certificate Transparency (CT) — a network of public, append-only, cryptographically-verifiable logs that Certificate Authorities are required to publish to. This tool queries crt.sh, a free search engine over those logs, for every certificate that has ever listed a name ending in your domain — surfacing subdomains you may not remember exist.
Why this matters for security
Attack surface you don't know about is attack surface you can't patch. A staging server spun up two years ago and forgotten, an internal admin panel that got a certificate "just in case," or a marketing microsite from a campaign that ended — all of these accumulate over time, and each one is a potential entry point running outdated software nobody's watching. Subdomain enumeration via CT logs is one of the very first steps in both professional security audits and, less comfortingly, in real attacker reconnaissance — it costs nothing and requires zero interaction with your infrastructure.
A name showing up here doesn't mean it's live
CT logs are permanent. A certificate issued for old-staging.example.com in 2019 stays in the log forever, even if that server was decommissioned the following month. Treat every result here as "a name that once had a certificate," not "a server currently running" — the next step is checking which of these actually resolve and respond today.