The OAuth2 protocol
If you are reading this book, you are in all likelihood someone who has logged in to a web page with a username and password. It's a straightforward model to confirm who you are, but there are drawbacks.
Many different websites exist, and each needs to properly handle someone's identity and password. The potential for security leaks multiplies with the number of different places an identity is stored, and how many routes a password can take through the different systems involved. It also becomes easier for attackers to create fake sites, as people become used to entering their username and password in multiple different places that may all look slightly different. Instead, you have probably come across websites that let you "Login with Google," Microsoft, Facebook, or GitHub. This feature uses OAuth2, or tools built on top of it.
OAuth2 is a standard that is widely adopted for securing web applications and their interactions with users...