Since the dawn of hacking, there have been lots of techniques released to backdoor Windows systems. From the wild wild west of the registry to the Windows Task scheduler, hackers love to find creative ways to maintain access to their conquests. While backdoors do not have to be related to malicious code, such as the case of an maliciously added user, often times a backdoor is thought to be synonymous with other sub-forms of malware. It just isn’t true… a backdoor can be much more interesting than a RAT hidden in the Run key. The Shmoocon 2013 talk by Jake Williams and Mark Baggett really hit home for me. The blog about this talk can be found here.
Taking a step back, lets pull straight from the most authoritative source on the internet… Wikipedia:
A backdoor in a computer system (or cryptosystem or algorithm) is a method of bypassing normal authentication, securing unauthorized remote access to a computer, obtaining access to plaintext, and so on, while attempting to remain undetected
Across the spectrum of assessments I go on, I find the concept to be not quite that simple. Backdoors possess multiple qualities that are worth breaking down to understand the decision points you have available to you as an operator. Further, there is no worse feeling than having your chosen backdoor be your single point of failure.