A system and method for authenticating an unauthenticated file level protocol using a sidecar authentication mechanism. A client transmits an authentication ticket, UID and list of network addresses to an authentication daemon of a storage system. The authentication daemon verifies the user identity and generates a file system credential that is stored in a cache indexed by an authentication tuple. Received data access operations from a client are compared to authentication tuples by UID and network address and the file system utilizes the stored credential for processing the data access operation.