An access control application for mobile devices is provided. The access control application may be configured to generate a set of security tasks based at least in part on information corresponding to a user's interactions with the mobile device. An unlock screen of the mobile device may be triggered and security tasks from the generated set of security tasks may be displayed through a user interface of the mobile device. The user's response to the security tasks may be obtained and a confidence score may be calculated, based at least in part on the response. The access control application may then determine, based at least in part on the score and one or more attributes of the environment, whether to unlock the mobile device or prompt the user to provide an additional response to another security task.