Private military and security companies (PMSCs) are increasingly contracted to provide security in international peacekeeping missions. Yet, we know very little about the practical implications of this development. How do PMSCs reinforce and shape security management within UN peacekeeping operations, and what are the consequences for UN missions and their host populations? To answer these questions, we explore the operational, representative and regulatory security practices in the UN operation in the Democratic Republic of Congo (MONUSCO). Our findings show how seemingly uncontroversial, even benign security practices can have unintended negative consequences. Specifically, we observe that the participation of security firms in MONUSCO's security management contributes to three developments: the differentiation of security between staff and locals, the hardening of MONUSCO's security posture, and the perpetuation of insecurity through the emergence of a local security economy. Contracted security is thus involved in reproducing forms of security that are in some ways diametrically opposed to the aims of the mission to protect civilians and facilitate a sustainable peace.