The invention relates to a portable authentication token comprising connection means for connecting to a computer, browser communication means for communicating with a browser running on the computer, and user authentication means for authenticating a user of the token to a server. The user authentication means are triggered via the browser communication means when the user connects to the server from the browser of the computer. The user authentication means are set to authenticate the user by communicating with the server through the browser. The token comprises out-of-band token communication means set to validate user authentication by establishing a communication channel between the token and the server, the communication channel bypassing the browser.The invention also relates to an authentication method and to a system comprising a token, a computer and a server to which the user authenticates with the token.