back door
noun /ˌbæk ˈdɔː(r)/
  /ˌbæk ˈdɔːr/
 Idioms - the door at the back or side of a buildingTopics Houses and homesb1
Idioms 
by/through the back door 
- in an unfair or indirect way- He used his friends to help him get into the civil service by the back door.