Backdoors arrive by different routes: installed by an attacker after compromise, planted by malware, embedded by a malicious insider or supplier, or left — intentionally or not — by developers as undocumented maintenance access. The defining property is circumvention: whoever uses the path does not pass through the controls everyone else must.
Detection depends on what the backdoor bypasses. Covert accounts, implanted services, modified binaries, rogue remote-access tools, and undocumented interfaces each leave different evidence — configuration artifacts, unexpected listeners, unexplained sessions — and none of it appears in the authentication path being skipped.
Key points
Common formsImplant components, covert local or domain accounts, unauthorized remote-access tooling, modified authentication code, vendor or developer maintenance paths, and supply-chain-planted mechanisms.
DetectionInventory accounts, services, listeners, and remote-access tooling against approved baselines; investigate sessions and administrative actions that lack a matching authentication trail.
RemovalIdentify how the path was created before closing it — deleting the account or tool without addressing the entry vector leaves the mechanism that rebuilds it.
Important limitationFinding one backdoor does not establish the full scope of a compromise, and a legitimate remote-access tool abused by an attacker is not automatically a “backdoor” — the unauthorized use is the violation, not necessarily the software.