OAuth (Open Authorization) is an open standard protocol that allows applications to securely access resources on behalf of a user without exposing the user’s credentials, such as passwords. It is commonly used for granting third-party applications limited access to a user’s protected resources, often in the context of modern web and mobile applications.

Here’s an overview of how the OAuth protocol works:

1. Client Application: The client application is the third-party application or service that wants to access a user’s resources on a resource server, such as a social media platform, cloud storage service, or an API.
2. Resource Owner: The resource owner is the user who owns the protected resources. These resources can include data, photos, contacts, or any other information stored on the resource server.
3. Authorization Server: The authorization server is responsible for authenticating the resource owner and granting authorization to the client application. It issues access tokens to the client after successful authorization.
4. Resource Server: The resource server hosts the protected resources that the client application wants to access. It verifies access tokens and grants or denies access to the requested resources.

The OAuth protocol involves several steps:

Step 1: Authorization Request

• The client application initiates the process by redirecting the user to the authorization server’s authorization endpoint. It includes information about the scope of access it is requesting.

Step 2: User Authorization

• The resource owner (user) interacts with the authorization server to grant or deny access to their resources. They may need to log in and provide consent to the requested actions.

Step 3: Authorization Grant

• If the user approves the request, the authorization server issues an authorization grant (often a temporary code) to the client application.

Step 4: Access Token Request

• The client application uses the authorization grant to request an access token from the authorization server by sending a secure HTTP POST request to the token endpoint.

Step 5: Access Token Response

• The authorization server validates the request, generates an access token, and sends it back to the client application.

Step 6: Accessing Resources

• The client application uses the access token to make authenticated requests to the resource server’s API. The access token serves as proof of authorization.

Step 7: Resource Server Validation

• The resource server validates the access token to ensure it is still valid and that the client has the necessary permissions to access the requested resources.

OAuth provides a standardized way for users to grant limited permissions to third-party applications without exposing their credentials. It is widely used for enabling single sign-on (SSO), allowing users to log in to multiple services using their social media or identity provider accounts.

Different OAuth versions exist, with OAuth 2.0 being the most widely adopted and offering improved security features compared to its predecessor, OAuth 1.0a. OAuth 2.0 is commonly used for web and mobile application authentication and authorization.