From their site: You build great web applications. We have millions of users who store their data on Yahoo!. Browser-Based Authentication (BBAuth) makes it possible for your applications to use that data (with their permission). BBAuth also offers a Single Sign-On (SSO) facility so that existing Yahoo! users can use your services without having to complete yet another registration process. The first time a user visits your web site, you redirect them to a specially constructed Yahoo! URL where they can login and grant your application permission.
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There are no source code libraries or how-to links for Yahoo BBauth. If you know one why not add it?
While Yahoo! may authenticate a user, the API does not tell you who the user is (their Yahoo! ID). This makes it difficult to grant or prevent access in your application if you don\'t know who the users are.