When implementing Role-Based Access Control (RBAC) in APIs, authorization is crucial to ensure that only authorized individuals have access to specific data. Authorization defines what actions and data a user or system is allowed to access. It is a key component of RBAC, ensuring that users are granted appropriate permissions and roles to perform their tasks while maintaining data security and integrity.

One of the benefits of using OpenID Connect is that it can help in eliminating the need for password storage. OpenID Connect allows for secure authentication without the application having to store user passwords. It achieves this by enabling Single Sign-On (SSO) and delegating the authentication process to an Identity Provider (IdP), reducing the security risks associated with password storage.

Comprehensive API documentation for a RESTful API should include sections on API endpoints, detailing all available endpoints with their URLs, request/response formats, and usage examples. Authentication details, including how to authenticate, obtain API keys or tokens, and code samples, should be provided. Rate limiting information, error codes, and explanations should be included. Additionally, a historical changelog with API changes and versioning information can be helpful for developers.

An API key is typically passed in an HTTP request as a request header. This method of transmission is more secure than other options, as it keeps the API key hidden from the URL and request body. It is common practice to include the API key in the headers of the HTTP request for authentication and authorization purposes.

In Role-Based Access Control (RBAC), access permissions are associated with roles rather than individual users. Roles define sets of permissions that can be assigned to users, making it more efficient to manage access control in systems with multiple users and varied access levels.

If an API key is accidentally leaked, the best practice is to immediately generate a new API key and update it in the application and relevant services. This prevents unauthorized access and potential security breaches. Continuing to use the same key without taking action could lead to security risks. Contacting the API provider may also be necessary, but the primary step is to update the key on your side.

The "access token" in OAuth 2.0 is a short-lived token used for user authentication. It provides the client (application) with limited access to a protected resource on behalf of the resource owner (user) after the user has granted permission. This token is crucial for securing APIs and ensuring authorized access to resources.

Swagger is a popular tool for generating interactive API documentation. It allows developers to view and test APIs in an easy-to-understand format. Microsoft Excel and paper-based manuals are not suitable for interactive API documentation. A text editor can be used for creating documentation but wouldn't provide the same interactive features as Swagger.

One of the challenges in automated API testing is ensuring that the tests are robust and easy to maintain. Robust tests can withstand changes in the API without frequent updates, making them sustainable in the long term and reducing maintenance efforts. Robust tests are less prone to break when the API evolves.

In the context of an SQL Injection vulnerability, an attacker can exploit it by injecting malicious SQL queries to access, modify, or delete data. To mitigate it, you should implement input validation, use prepared statements or stored procedures, and consider using a Web Application Firewall (WAF) to filter out malicious input. These measures help prevent SQL Injection attacks.