Learn in the textbookMalware Variants and Specialized Types
DefinitionA backdoor (バックドア) is a secret entrance an attacker sets up to get into a system without passing normal authentication.
ExplanationIt is often built by an attacker who has already broken in and wants an easier way back, by planting a program or changing settings. The entrance itself is inconspicuous, so it may be used again and again without anyone noticing.
AnalogyIt is like a burglar who, on the first break-in, cuts a spare key for the back door. After that they can come and go without the front-door key.
Memory hook"Back door" means just that. Remember it literally as an entrance that avoids the front door, the normal login.