**localhost Subdomains and OAuth: A Development Conundrum**

Developers may be familiar with using localhost subdomains, such as app.localhost, to access the 127.0.0.1 address in their browsers. However, a recent discovery highlights an unexpected challenge when attempting to combine these subdomains with OAuth. According to reports, it is currently impossible to make localhost subdomains work seamlessly with OAuth.

**The Challenge**

The issue arises from restrictions on callback URLs, which must be set to http://localhost. This limitation prevents the use of shared callback URLs for multiple subdomains under the same domain (e.g., foo.localhost). As a result, developers are left without viable solutions, forcing them to consider paid options.

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Source: https://dev.to/christopher_oezbek/mission-impossible-with-localhost-8dj