Security in applications and operating systems is a critical aspect in an era where cyber threats are continuously evolving in number and complexity. Its main aim is to safeguard data, ensure system integrity, and protect against unauthorized access. In applications, security focuses on implementing measures such as encryption, authentication, and secure coding practices to prevent vulnerabilities, like data breaches or malware infections. To ensure effective security mechanisms it is important to understand the different malware stages, as well the different categories of malware. On the other hand, operating system security, involves enforcing access controls, managing user permissions, and applying protective mechanisms, as antivirus. Together, application and OS security are crucial for a robust defence strategy, ensuring that both software and system resources are protected from known threats and zero-day attacks.