Which type of malware hides the existence of other malware by modifying the underlying operating system?

The type of malware that hides the existence of other malware by modifying the underlying operating system is known as a rootkit. A rootkit is a collection of software tools that an unauthorized user can use to gain control over a computer system without being detected. It achieves this by altering the operating system's functionality or disguising itself and other malicious software, making it difficult for security software to identify or remove these threats.

Rootkits typically operate at a low level within the system, often integrating themselves into the kernel, which is the core part of the operating system. This level of access allows them to conceal their presence and the presence of other malicious programs, ensuring that malicious activity remains undetected by normal security measures.

In contrast, viruses, worms, and Trojan horses have different functions and characteristics. For example, viruses attach themselves to legitimate programs and spread when those programs are run, while worms replicate themselves across networks without needing a host program. Trojan horses disguise themselves as legitimate software to trick users into installing them but do not specifically focus on concealing other malware. This distinction highlights why rootkits are uniquely capable of hiding malware effectively.