Mixed-Mode Authentication

Techniques for mixed-mode authentication are described. In one or more embodiments, an authentication service may be implemented to selectively configure and issue authentication tokens based upon an optional secure mode that enables enhanced security. Clients may be provided with an option to choose between an insecure mode and a secure mode for authentications. Based on this choice, tokens may be configured to include an indication of whether the secure mode is disabled or enabled. When secure mode is disabled, an insecure token valid for both secure sites and other sites is issued to a client when the client is authenticated. When the optional secure mode is enabled, both secure and insecure tokens are provided to the client. The authentication services and/or other services may be configured to reject an insecure token when secure mode is enabled to prevent unauthorized use of a stolen token to access secure resources.