ELI5: How do hackers find/gain 'backdoor' access to websites, databases etc.?
Gunna try doing this like ELI10. Back door access is just a way of saying “not-expected"access. Sometimes its still done through the front door, and sometimes its through a window.
Something like the front door would be if your Mom told you you could have one glass of coke, and you went and got the big glass flower vase, and poured 6 cokes into it. By following the rules in an unexpected way, you’ve tricked the machine. When mom asks you later how many glasses of coke you had, (of course with her trusty polygraph), you can truthfully answer, "One”. This might be like an SQL injection. Instead of answering 5+8=__ with “14”, you might answer with “14&OUTPUT_FINAL_ANSWER_LIST”. Since it has no spaces and starts with numbers, it might satisfy the rules.
Another way would be if your Mom said you could invite some friends over to play. After the 5th friend walks in, your Mom declares, “That’s it, not another kid walks through that door!” If you open a window and let Johnny climb in with his crayons, technically you didn’t break the rules (for the eventual polygraph) AND when you and your 5 friends go downstairs for homework, Johnny can color all over the walls without someone suspecting he’s there. This is as though you made new login names and used one of the names to give another person administrative, or Mommy, rights. Sometimes you need to make a new login screen, or just knock open a hole in the wall and cover it with a poster, but the idea is still to break the intention of the rules while following them to the letter.
What’s also important to remember is this goes very smoothly when someone lives in the house already, but becomes much harder when you’re trying to get into a stranger’s house. You might have to try to sell them cookies or magazines and then write down where the windows are. Or you might have to offer to clean their whole house for only $5, and then leave a window unlocked for your friend to come back later. Getting inside is a major step.