Flickr is an image and video hosting website which is owned by Yahoo and resides on the flickr.com domain. To handle authentication on Flickr, requests are made to login.yahoo.com to get an access token for the user.

http://blog.mish.re/index.php/2017/04/29/yahoo-bug-bounty-chaining-3-minor-issues-to-takeover-flickr-accounts/