Recent advances in data protection regulations brings privacy benefits for website users, but also comes at a cost for operators. Retrofitting the privacy requirements of laws such as the General Data Protection Regulation (GDPR) onto legacy software requires significant auditing and development effort. In this work we demonstrate that this effort can be minimized by viewing data protection requirements through the lens of information flow tracking. Instead of manual inspections of applications, we propose a lightweight enforcement engine which can reliably prevent unlawful data processing even in the presence of bugs or misconfigured software. Taking GDPR regulations as a starting point, we define twelve software requirements which, if implemented properly, ensure adequate handling of personal data. We go on to show how these requirements can be fulfilled by proposing a metadata structure and enforcement policies for dynamic information flow tracking frameworks. To put this idea into practice, we present Fontus, a Java Virtual Machine (JVM) information flow tracking framework, which can transparently label personal data in existing Java applications in order to aid compliance with data protection regulations. Finally, we demonstrate the applicability of our approach by enforcing data protection polices across 7 large, open source web applications, with no changes required to the applications themselves.