The invention relates to a method and system for authenticating a user of a data transfer device (such as a terminal in a wireless local area network, i.e. WLAN). The method comprises: setting up a data transfer connection from the data transfer device to a service access point. Next, identification data of the mobile subscriber (for example an MSISDN) are inputted to the service access point. This is followed by checking from the mobile communications system whether the mobile subscriber identification data contains an access right to the service access point. If a valid access right exists, a password is generated, then transmitted to a subscriber terminal (for example a GSM mobile phone) corresponding to the mobile subscriber identification data, and login from the data transfer device to the service access point takes place with the password transmitted to the subscriber terminal.