What type of malware is exemplified by a small program called Mitglieder that allows outsiders to infiltrate a user's machine?

The example of malware known as Mitglieder that allows outsiders to infiltrate a user's machine best fits the definition of a Trojan horse. A Trojan horse is typically disguised as legitimate software, tricking users into installing it on their systems. Once installed, it can grant unauthorized access to the attacker's control without the user's knowledge. This behavior aligns with the function of Mitglieder, as it allows external entities to infiltrate the user's machine under the guise of a useful program.

In contrast, a virus attaches itself to a host program and replicates, requiring user action for distribution. A worm independently replicates itself across networks without needing a host file, spreading through vulnerabilities in systems. Spyware, while also malicious, primarily focuses on gathering sensitive information from a user’s machine rather than facilitating infiltration or remote control by a third party. Thus, the characteristics of Mitglieder align specifically with those of a Trojan horse.