Sanctuary cities worldwide often claim to support precaritised migrants residing in their jurisdiction as a reaction against exclusionary national policies. This paper is the first of its kind to analyse how digital technologies hinder the efficacy of sanctuary policies, in a way that may render them obsolete. Drawing on comparative evidence from the UK and Canada, it explores digitally-driven responses to the COVID-19 pandemic by different government levels (local, regional, national) and their impact on migrants rights. Findings reveal that the increasing interoperability among population databases have crucially enhanced the capacity of immigration authorities to access sensitive data collected by local service providers, which can then be used to detect, detain, and deport precaritised migrants. Such practices of hostile data-sharing thus weaken pre-existing sanctuary protections that are based on limited cooperation among local and national officials. Yet, local actors have sometimes deployed fresh counter-strategies, notably building non-interoperable data management infrastructures so as to ensure safer access to basic healthcare services. While prior scholarship has mostly examined the role of digitisation in external bordering processes, this paper adds to the academic debate to the domain of internal borders.