What type of malware is characterized by disguising itself within seemingly harmless files to access a system?

The type of malware that is characterized by disguising itself within seemingly harmless files to access a system is a Trojan horse. A Trojan horse typically presents itself as a benign piece of software, such as a game, application, or file, tricking users into installing it. Once installed, it can execute malicious activities without the user's knowledge.

The nature of a Trojan horse relies heavily on social engineering; users are often deceived into believing that they are downloading something safe or useful. This deception is key to its operation, as it does not replicate itself like a virus or worm. Instead, it relies on user intervention to infect a system, making it particularly insidious.

In contrast, a virus attaches itself to legitimate programs and can replicate itself, while a worm spreads across networks independently without needing user action. Ransomware, on the other hand, encrypts files and demands payment for the decryption key but does not disguise itself as something harmless. Therefore, the ability of a Trojan horse to masquerade as a harmless file makes it unique among these types of malware.