A backdoor is a type of malicious code that, once installed on a system, allows attackers to bypass normal security access controls and access the system.
Also called a trapdoor. An undocumented way of gaining access to a program, online service, or an entire computer system. A programmer who creates a threat program writes the code for a backdoor, as well. Often only the programmer knows the backdoor details. A backdoor is a potential security risk.