Today, power systems have transformed considerably and taken a new shape of geographically distributed systems from the locally centralized systems thereby leading to a new infrastructure in the framework of networked control cyber-physical system (CPS). Among the different important operations to be performed for smooth generation, transmission, and distribution of power, maintaining the scheduled frequency, against any perturbations, is an important one. The load frequency control (LFC) operation actually governs this frequency regulation activity after the primary control. Due to CPS nature, the LFC operation is vulnerable to attacks, both from physical and cyber standpoints. The cyber-attack strategies ranges from a variety of attacks such as jamming the network communication, time-delay attack, and false data injection. Motivated by these perspectives, this paper studies the cybersecurity issues of the power systems during the LFC operation, and a survey is conducted on the security analysis of LFC. Various cyber-attack strategies, their mathematical models, and vulnerability assessments are performed to understand the possible threats and sources causing failure of frequency regulation. The LFC operation of two-area power systems is considered as a tutorial example to quantify the vulnerabilities. Mitigation strategies through control theoretic approaches are then reviewed and highlighted for LFC operation under cyber-attack.