This paper is about the work involved in modifying firmware images with the test case focused on Cisco IOS. It will show how it is a common misconception that doing such a thing involves advanced knowledge or nation state level resources. This paper provides sound methodologies, shows how to approach the subject, and walks the reader through the entire process while providing the necessary knowledge so that by the end of the paper, if the reader is to follow it completely through, they will have a basic but functional firmware rootkit.
You can download it from the following link: https://packetstormsecurity.com/files/download/133917/cisco_ios_rootkits.pdf
Source: https://packetstormsecurity.com/files/133917/Writing-Cisco-IOS-Rootkits.html