With the ever-increasing demands of multimedia services and the boom of smartphones, traditional online multimedia applications are extended to mobile users anywhere and anytime. However, the flourishing of multimedia services is still hindered by inherent security and privacy concerns. In this article, we investigate the security and privacy issues of multimedia services by studying a newly emerging multimedia-oriented mobile social network (MMSN), which helps users receive multimedia services not only from their online social communities but also from their social friends in the vicinity. Specifically, we first define the MMSN architecture, and identify the unique security and privacy challenges. Then we study three MMSN applications: content query, service evaluation, and content filtering. For each application, we present the specific security and privacy problems with the corresponding countermeasures. Finally, we propose some future research directions in the MMSN.