Ask HN: Online embedded Linux security training?

My boss has suggested for me to attend a Doulos Embedded security course[1] but I don’t want to travel.

Any suggestions for some online courses (free or paid) I could do? Any experiences with learning about security you are willing to share?

We make embedded Linux products that run a lot of legacy code written in C/C++ that have huge security holes. I need to learn how to close the holes and improve our design practices for future development.

[1] https://www.doulos.com/content/training/emblinux_security_comprehensive.php