In order to learn how operating systems work on a x86 chip, I had written (long ago) a toy OS based on code found on osdev wiki . It allowed me to understand the interaction between the OS and the x86 hardware, and to some extent how the OS handles userspace (I used newlib for some userspace programs). The single-processor OS code later served me well as a base for a course about operating systems I taught at the university.
D-Bus is a easy to use message-oriented mechanism that allows communication between multiple processes running concurrently on the same machine. Any application running on the session bus can install a monitor hook over the bus to listen to all exchanged messages. Sensitive information such as mail passwords will be exchanged in plain text. A malicious software can easily listen and parse all those sensitive messages.