Backdoor deployment is the act of placing unauthorized access on a server or application so an attacker can return later without going through normal authentication. A backdoor may be a hidden account, a web shell, a modified service, a scheduled task, or another persistence mechanism that survives basic cleanup.
This matters because it turns a one-time compromise into ongoing control. In cyber security, backdoors are used after initial access to maintain footholds, reload malware, steal data, or change system settings again later. On servers that manage software, repositories, or build pipelines, a backdoor can also threaten trust in the files other systems download and use. Defenders look for unexpected users, new files, altered startup items, suspicious admin activity, and network connections that do not match normal administration. Removing the visible exploit is not enough; incident response must also verify that no hidden access remains.