Click to access Firmware_Security_Testing_Methodology_Version1.pdf
Note there is a companion VM:
https://github.com/scriptingxss/EmbedOS
Hmm, I can only find the PDF version of this content, no HTML/wiki version… See-also:
https://www.owasp.org/index.php/OWASP_Embedded_Application_Security
https://scriptingxss.gitbook.io/embedded-appsec-best-practices/
https://www.owasp.org/index.php/OWASP_Internet_of_Things_Project