Security Guide

An attack can also include the implementation of a backdoor, so an attacker can gain unauthorized access to the system at a later time (undetected and with bypassing TYPO3 authentication mechanisms). Even if the security vulnerability was fixed (entry point secured) and all passwords changed, the attacker would be able to harm the system in the future.

Fix: mention the possibility of backdoors after an attack in chapter "Detect, Analyze and Repair a Hacked Site".