Which authentication type is NOT supported by custom connectors?

Custom connectors in Microsoft Power Automate support several types of authentication to ensure secure access to APIs. Among the authentication methods listed, Cookie Authentication is typically not supported.

The reason for this is that cookie-based authentication relies on maintaining a session state on the server-side, which does not align well with the stateless nature of API interactions. Custom connectors are designed for scenarios where access to services is generally stateless and often involves token-based or key-based mechanisms.

In contrast, options like Anonymous, API Key, and OAuth 2.0 are commonly used in custom connectors. Anonymous authentication allows access without any credentials, which is applicable in cases where security is not a concern. API Key authentication provides a simple mechanism for using a key that gives access to resources, which fits well within the structure of API requests. OAuth 2.0 is one of the most popular and secure methods for authorization, allowing users to grant third-party applications limited access to their resources without exposing their credentials.

Thus, while the other authentication types are feasible and supported by custom connectors, Cookie Authentication is not a suitable choice for the requirements of custom connectors in Power Automate.