1. A system to provide authentication services to legacy applications, the system comprising: one or more legacy applications executing on an application server and configured to authenticate a user based on one or more user credentials received from a client using a legacy authentication protocol, wherein the one or more legacy applications are not configured to authenticate using a Kerberos authentication protocol; an authentication proxy module executing on a computing device and configured to receive legacy authentication credentials from the one or more legacy applications corresponding to the legacy authentication protocol, wherein the legacy authentication credentials are associated with the one or more user credentials; and a credential binding module in communication with the authentication proxy module and configured to associate the legacy authentication credentials with a cached Kerberos credential, and wherein the authentication proxy module is further configured to: (i) authenticate the user corresponding to the legacy authentication credentials using a Kerberos authentication protocol by invoking a Kerberos authentication request to a Kerberos server, and (ii) use the cached Kerberos credential received from the credential binding module to authenticate the user when the Kerberos server is temporarily unavailable.