back door 
noun
1The rear door of a building.They pushed open the back doors and entered the building, passing by the control room and up the concrete back stairs, heading for their changing rooms....
- Bob pried open one of the back doors to the building, and they all scrambled out into the cool night air.
- Veon threw open his door and opened the back doors, grabbing his pistols and stuffing them into his belt.
1.1 [as modifier] Achieved by using indirect or dishonest means: a back-door tax increase...
- The problem with such a ‘solution’, of course, is that it is a slightly dishonest, back-door way of achieving a result which accords with justice.
- It is wrong, it is a back-door tax, and we totally disagree with it.
- That is why the National Party opposes this ridiculous back-door tax.
1.2A feature or defect of a computer system that allows surreptitious unauthorized access to data.Around the same time, Mydoom.A was infecting machines around the world, leaving a small backdoor to each infected computer....
- Once downloaded, the victim unwittingly activates the backdoor by compiling Sendmail from source code.
- It also attempts to open a backdoor on infected Windows PCs, allowing hackers to exploit compromised systems.
Phrases
by (or through) the back door