Flexible Authentication: Reimagining authentication for millions of users at Airbnb

This post details the architectural decisions and results of rebuilding Airbnb's authentication flows using a server-driven 'Identify first then Challenge' model. It introduces the 'Challenge Picker' for fallback options and a fully server-driven screen abstraction, which decouples authentication strategy from client releases. The post highlights a 60% reduction in client code, a 100KB reduction in the web client bundle, an increase in successful authentications by 2.6%, a 27% reduction in duplicate accounts, and an 11% reduction in OTP costs.