Censys vs FilterLists

Censys grew out of the University of Michigan's ZMap internet-scanning research and now re-scans the full public IPv4 space multiple times a day, fingerprinting open ports, services, and TLS certificates. Its Search API exposes that host and certificate-transparency dataset for asset discovery, exposure monitoring, and threat hunting, and underpins Censys's own attack-surface-management product. Access is account-scoped through an API key, with a limited free community tier and metered paid plans for higher query volume.

FilterLists is Collin Barrett's independent, actively-maintained directory of filter and host lists used by adblockers, firewalls, and DNS sinkholes to block ads, trackers, malware, and other unwanted content. Its ASP.NET Core REST API serves the same catalog that powers the site -- filter lists, the domains/URLs they target, licenses, and maintainer info -- with an interactive Swagger UI for exploring the schema before writing a query. The API and site are fully free and keyless, open-sourced on GitHub with commits as recent as this week. The API lives entirely on a separate `api.filterlists.com` host -- the intuitive REST paths on the main `filterlists.com` domain just return the site's own SPA shell.