owasp.org
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OWASP is a nonprofit devoted to improving software and web application security, offering free, open resources such as the OWASP Top Ten, secure coding guidelines, testing guides, and community projects.
More resources on Operating System Security
Kernel Documentation: Security
Official Linux kernel security docs
Grsecurity
Linux kernel security enhancements
SELinux Project
Mandatory Access Control for Linux
LiveOverflow: Introduction to Binary Exploitation
Series on OS-level security exploits and mitigations
Computer Systems Security
Explore operating system security with Nick Weaver's Computer Systems Security course. Learn to build robust, secure systems!
geeksforgeeks.org
GeeksforGeeks is a comprehensive computer science learning site offering tutorials, explanations, and coding practice across topics including operating systems and security. It features step-by-step guides, examples, and interview-ready articles on OS concepts, security fundamentals, and related programming topics.