The invention relates to a method and system for authenticating a user, wherein an application transmits a query comprising identification data to an authentication service, the authentication service determines the address of a mobile terminal linked to the user on the basis of the identification data and transmits a request comprising a transaction identifier to the mobile terminal, the mobile terminal performs an enquiry for input of a biometric security feature, grants access to a private key saved on the mobile terminal upon input of a valid security feature, signs the transaction identifier using the private key and transmits the signed transaction identifier back to the authentication service, and the authentication service verifies the signature of the signed transaction identifier and, in the case of the presence of an authentic signature, transmits a confirmation of the query back to the application.