Best type of back door hack, is to first come in via the back door ... then find front door vulnerabilities, and hack through them.  That way, you don't bring attention to the back door access, and preserve it for future use.
Open source software is probably the best defense against back door attacks.  I use Linux (Ubuntu MATE) on my PC.  Soon as anyone establishes a back door in any flavor of Linux, the millions of programmers in the Linux community spots, and eradicates it.  Proprietary software (Windows) can be guarded only by the few programmers who have the keys to the encryption.