IT-Security Researchers from the University of Vienna and SBA Research identified and responsibly disclosed a large-scale privacy weakness in WhatsApp's contact discovery mechanism that allowed the enumeration of 3.5 billion accounts. In collaboration with the researchers, Meta has since addressed and mitigated the issue. The study underscores the importance of continuous, independent security research on widely used communication platforms and highlights the risks associated with the centralization of instant messaging services. The preprint of the study has now been published, and the results will be presented in 2026 at the Network and Distributed System Security (NDSS) Symposium.