Fixes #4066 (#5094)

free-courses-en.md

@@ -647,6 +647,7 @@
 
 ### Security
 
+* [Computer Systems Security](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-858-computer-systems-security-fall-2014) - Prof. Nickolai Zeldovich (MIT OpenCourseWare)
 * [Ethical Hacking](https://www.hacker101.com/videos) -(Hacker101)
 * [The Complete Cyber Security & Hacking Course](https://academy.ehacking.net/p/cyber-security-training-hacking-course) -(Insec Techs- EH Academy)
 * [Zero to Hero: A Practical Network Penetration Testing Course](https://www.youtube.com/playlist?list=PLLKT__MCUeiwBa7d7F_vN1GUwz_2TmVQj) - The Cyber Mentor