Final answer:
Rootkits are the technology that allow a hidden backdoor to be used for access to workstations on the Internet, providing remote control to unauthorized users while remaining undetected.
Step-by-step explanation:
The technology that allows a hidden backdoor to be used for access to workstations on the Internet is Rootkits. Rootkits are a type of malicious software that can give an unauthorized user remote control of a computer system without being detected.
Rootkits typically alter parts of the operating system or install themselves as drivers or kernel modules to remain hidden. Once installed, attackers can perform various malicious activities, such as stealing data, installing additional malware, or controlling the system remotely.
Other options like firmware can contain vulnerabilities exploited for unauthorized access, but they are not specifically meant for creating backdoors. SQL injection and Cross-site scripting (XSS) are types of attacks against web applications rather than methods for establishing a persistent, hidden backdoor.