This paper presents a comprehensive survey of security and privacy challenges in Sixth-Generation (6G) and future communication networks; moving beyond fragmented standards and technology-specific studies to deliver a cross-layer analysis of evolving threats and countermeasures. Unlike previous generations, 6G will integrate diverse enabling technologies such as Reconfigurable Intelligent Surfaces (RIS), Joint Sensing and Communication (JSAC), Non-Terrestrial Networks (NTN), and AI-native architectures, significantly expanding the attack surface of communication networks. We examine vulnerabilities due to new enabling technologies, new architectural features, and new applications which are expected in 6G networks, and highlight risks including adversarial machine learning, software supply-chain attacks, privacy breaches, and quantum-era threats. By synthesizing insights from cutting-edge research literature and standardization/pre-standardization bodies such as 3GPP, ETSI, ITU, IETF, O-RAN Alliance, AI-RAN Alliance, we develop a unified threat taxonomy and map emerging solutions, including zero-trust frameworks, blockchain-based authentication, quantum-safe cryptography, and privacy-preserving edge intelligence. This survey offers researchers and practitioners a holistic foundation for designing secure-by-design, resilient 6G architectures that address interdependent, multi-layer risks in future hyper-connected ecosystems.