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crt.sh
Certificate transparency log search
Overview
A free web front-end over the public Certificate Transparency logs. Search a domain and get every certificate ever issued for it and its subdomains — which, because CT logging is mandatory for public CAs, is one of the most reliable passive subdomain sources available.
Field Note
Wildcard and SAN certificates mean you will see subdomains that were only ever used briefly, or internal names accidentally included on a public cert — worth cross-referencing against DNS to confirm what is still live. The site occasionally queues under load; the same data is available via a documented JSON API (`?q=domain&output=json`) if you are scripting it.
- Best for
- The single most reliable free source for passive subdomain discovery.
- Pricing
- Free, no account required
- Last updated
- August 17, 2026