5G NR sidelink communication enables new possibilities for direct device-to-device interactions, supporting applications from vehicle-to-everything (V2X) systems to public safety, industrial automation, and drone networks. However, these advancements come with significant security challenges due to the decentralized trust model and increased reliance on User Equipment (UE) for critical functions like synchronization, resource allocation, and authorization. This paper presents the first comprehensive security analysis of NR V2X sidelink. We identify vulnerabilities across critical procedures and demonstrate plausible attack, including attacks that manipulate data integrity feedback and block resources, ultimately undermining the reliability and privacy of sidelink communications. Our analysis reveals that NR operational modes are vulnerable, with the ones relying on autonomous resource management (without network supervision) particularly exposed. To address these issues, we propose mitigation strategies to enhance the security of 5G sidelink communications. This work establishes a foundation for future efforts to strengthen 5G device-to-device sidelink communications, ensuring its safe deployment in critical applications.