"develop rootkits for the .NET framework,[...]
Code reviews will not detect backdoors installed inside the Framework since the payload is not in the code itself, but rather it is inside the Framework implementation.
Writing Framework rootkits will enable the attacker to install a reverse shell inside the framework, to steal valuable information, to fixate encryption keys, disable security checks and to perform other nasty things [...]"
Download, Userguide, Source Code, etc. linked below.