Which type of attack is characterized by overwhelming a system with nonlegitimate service requests?

A denial of service attack is characterized by overwhelming a system with nonlegitimate service requests, effectively rendering the system unable to respond to legitimate requests. This type of attack exploits system vulnerabilities by flooding the target with excessive traffic or requests, consuming its resources, and causing service interruptions or outages. The intent behind this attack is to disrupt the normal operations of a service, making it unavailable to users who need to access it.

In contrast, a Trojan horse attack involves malicious software disguised as a legitimate program, which may facilitate unauthorized access after it is installed by the user. A privilege escalation attack focuses on exploiting system vulnerabilities to gain higher access rights than are typically allowed, which can lead to unauthorized actions but does not inherently involve overwhelming a system with requests. A brute-force attack methodically attempts to guess passwords or encryption keys, relying on sheer computing power rather than overwhelming service requests to disrupt a system. Each of these attacks has distinct characteristics and objectives, making denial of service the only option that directly matches the description of overwhelming a system with illegitimate requests.