die Hintertür
back door
A second entrance or exit, usually located at the back of a building.
Die Lieferanten benutzen immer die Hintertür.
The delivery people always use the back door.

back door / backdoor
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back door
A second entrance or exit, usually located at the back of a building.
Die Lieferanten benutzen immer die Hintertür.
The delivery people always use the back door.

The word 'Hintertür' (back door) is a simple compound made from the preposition 'hinter' (behind) and the noun 'die Tür' (the door). So it literally means 'the door that is located at the back.'
Wir haben den Müll durch die Hintertür nach draußen gebracht.
We took the trash outside through the back door.
The figurative meaning is used almost exclusively in the fixed expression 'durch die Hintertür' (through the back door). This idiom describes how someone achieves something by unofficial, indirect, or unfair means.
Sie hat versucht, die neuen Regeln durch die Hintertür einzuführen.
She tried to introduce the new rules through the back door.
In the field of information technology, 'Hintertür' (backdoor) is a technical term. It refers to secret access to a computer system that bypasses normal security checks. The context here is always technology, software, or cybersecurity.
Hacker nutzen oft eine Hintertür, um unbemerkt auf Systeme zuzugreifen.
Hackers often use a backdoor to access systems unnoticed.
detour
'Umweg' and 'durch die Hintertür' (through the back door) can both express that someone does not take the direct route. 'Umweg' can quite literally be a longer route to a place. It can also mean figuratively that someone reaches a goal indirectly or in a complicated way. 'durch die Hintertür', on the other hand, is a fixed expression and often sounds critical: someone achieves something in an unofficial or not entirely honest way. The words can be confused because both describe an indirect approach. But 'Umweg' is more neutral; 'durch die Hintertür' more strongly suggests tricks or getting around rules.

Er bekam die Stelle nicht offiziell, sondern durch die Hintertür.
He did not get the job officially, but through the back door.
Wegen der Baustelle mussten wir einen langen Umweg zum Bahnhof nehmen.
Because of the roadworks, we had to take a long detour to the train station.