We propose a system for continuous reauthentication of web users based on the observed mouse dynamics. Key feature of our proposal is that no specific software needs to be installed on client machines, which allows to easily integrate continuous reauthentication capabilities into the existing infrastructure of large organizations. We assess our proposal with real data from 24 users, collected during normal working activity for several working days. We obtain accuracy in the order of 97%, which is aligned with earlier proposals requiring instrumentation of client workstations for intercepting all mouse activity---quite a strong requirement for large organizations. Our proposal may constitute an effective layer for a defense-in-depth strategy in several key scenarios: web applications hosted in the cloud, where users authenticate with standard mechanisms; organizations which allow local users to access external web applications, and enterprise applications hosted in local servers or private cloud facilities.