# Backdoor

A hidden way into a system that bypasses normal authentication and security checks.

A **backdoor[↗](/backdoor)** is a hidden way into a system that bypasses normal security checks such as authentication[↗](/authentication) and authorization[↗](/authorization). It lets an attacker come back in secretly, whenever they want, even if the "front door" is locked.

Backdoors take different forms: a hidden access point planted by malware[↗](/malware) (such as a trojan[↗](/trojan)), a mechanism an attacker leaves behind for persistence after a compromise, or a flaw arising from careless or poor software design. Sometimes access left in place by developers for maintenance, and never cleaned up, unintentionally becomes a backdoor.

The existence of a backdoor means a system is at continuous risk, not a one-time incident, so simply removing the visible malware from a compromised system is not enough. Every persistence mechanism, added account, and altered configuration must be fully identified and removed, and often the safest approach is to rebuild the system from a clean source entirely.