Denial of service (DoS) is an attack that makes a system, network, or application unavailable by exhausting resources or triggering failure. Floods can consume bandwidth, connection state, CPU, memory, or request-processing capacity; a software defect may instead be exploited to crash a service. A distributed denial-of-service (DDoS) attack generates traffic from many systems, increasing volume and complicating source-based blocking. The material security impact is loss of availability for users and dependent services, including websites, APIs, DNS, and operational systems.
Mitigation should match the bottleneck. Rate limits, request validation, connection protections, caching, and service isolation can reduce application and state-exhaustion attacks; volumetric traffic generally needs filtering or absorption upstream of the network. Monitoring should distinguish abnormal request patterns from ordinary load and alert on saturation, while tested failover, traffic diversion, and restoration procedures limit outage duration. Vulnerability management and timely patching reduce DoS caused by remotely triggerable crashes, but do not replace capacity planning and resilience testing.