There are a number of cloak and dagger considerations that potentially come into play when apportioning responsibility.
Covert backdoors sometimes masquerade as inadvertent defects (bugs) for reasons of plausible deniability.
These backdoors can be inserted either directly in the on-disk object code, or inserted at some point during compilation, assembly linking, or loading – in the latter case the backdoor never appears on disk, only in memory.
As this requires subverting the compiler, this in turn can be fixed by recompiling the compiler, removing the backdoor insertion code.
This defense can in turn be subverted by putting a source meta-backdoor in the compiler, so that when it detects that it is compiling itself it then inserts this meta-backdoor generator, together with the original backdoor generator for the original program under attack.
The threat of backdoors surfaced when multiuser and networked operating systems became widely adopted.
Petersen and Turn discussed computer subversion in a paper published in the proceedings of the 1967 AFIPS Conference.
A backdoor may take the form of a hidden part of a program one uses, Although normally surreptitiously installed, in some cases backdoors are deliberate and widely known.
These kinds of backdoors might have "legitimate" uses such as providing the manufacturer with a way to restore user passwords.
Default passwords (or other default credentials) can function as backdoors if they are not changed by the user.
Some debugging features can also act as backdoors if they are not removed in the release version.
In some cases these might begin life as an actual bug (inadvertent error), which once discovered are then deliberately left unfixed and undisclosed, whether by a rogue employee for personal advantage, or with C-level executive awareness and oversight.
It is also possible for an entirely above-board corporation's technology base to be covertly and untraceably tainted by external agents (hackers), though this level of sophistication is thought to exist mainly at the level of nation state actors.
For example, if a photomask obtained from a photomask supplier differs in a few gates from its photomask specification, a chip manufacturer would be hard-pressed to detect this if otherwise functionally silent; a covert rootkit running in the photomask etching equipment could enact this discrepancy unbeknown to the photomask manufacturer, either, and by such means, one backdoor potentially leads to another.