A backdoor is a security risk that provides unauthorized access to an application. A backdoor is a hidden or undocumented way of bypassing the normal authentication or encryption mechanisms of an application, allowing an attacker to gain remote access, execute commands, or steal data. A backdoor can be created intentionally by the developer, maliciously by an attacker, or unintentionally by a programming error. Reference: CompTIA Server+ Certification Exam Objectives, Domain 5.0: Security, Objective 5.2: Given a scenario, apply logical access control methods.