Which characteristic of a denial-of-service attack makes it particularly concerning?

The characteristic of a denial-of-service (DoS) attack that makes it particularly concerning is its ability to reduce system performance. DoS attacks overwhelm a system, service, or network with a flood of traffic or requests, rendering it unable to function properly. This disruption can lead to legitimate users being unable to access services, causing significant downtime and potential financial loss for organizations.

While other options outline important security concerns, they do not define the core issue with DoS attacks. For instance, the encryption of data, which makes it inaccessible, is more closely associated with ransomware attacks rather than DoS attacks. Furthermore, the potential for data theft involves different attack vectors, such as malware or phishing, rather than the fundamental nature of a DoS attack. While some denials of service can be simple to initiate, the concern primarily centers on the impact it has on system performance and availability.