A system and method for authenticating users against an external directory service. A client device issues an LDAP (Lightweight Directory Access Protocol) request (e.g., a login request) to a local or native directory server (e.g., an Oracle Internet Directory server) configured to authenticate users for access to a resource (e.g., an Oracle database, an Oracle application server). The native directory server does not maintain or synchronize user passwords, and forwards the request (or details of the request) to a plug-in residing in the resource. The plug-in forwards or issues the request to an external or third-party directory server or service, which attempts to authenticate the user and returns a result indicating success or failure. The plug-in returns the result to the local server, which responds to the client.