Normative debates on migration control, while characterised by profound disagreement, do appear to agree that the state has at least a prima facie right to prevent the entry of security threats. While concern is sometimes raised that this ‘security exception’ can be abused, there has been little focus by normative theorists on concrete practices of security, and how we can determine what a ‘principled’ use of the security exception would be. I argue that even if states have a right to (...) prevent migrants from entering on grounds of security, the practices required to implement this right will likely compromise core liberal democratic values in ways that have not been sufficiently appreciated. I draw on literature from Science and Technology Studies and Critical Security Studies to examine the proliferation of digital border control technologies which are increasingly dominating state security practices, and explain the challenges these technologies pose to non-discrimination, liberty and equality. I also highlight the difficulties of taking these principles as presumptive limitations on the operation of the security exception. A more sustained conversation among ethical theorists is needed to address the profound ethical challenges that the ever-increasing use of these technologies poses, particularly in pursuit of security. (shrink)