What is a DoS attack?

A Denial of Service attack makes a service unavailable by overwhelming it with traffic or exploiting a vulnerability; a DDoS does this from many sources at once.

DoS = Denial of Service

A DoS attack aims to make a service unavailable by overwhelming it with traffic or exploiting vulnerabilities.

* A DDoS floods a target from many sources at once, overwhelming it so legitimate users can't reach it. *

Types:

• Volume-based: Flood the target with traffic
• Protocol attacks: Exploit protocol weaknesses (SYN flood)
• Application layer: Target specific services

DDoS = Distributed Denial of Service

• Attack comes from many sources
• Harder to block than single-source DoS

CIA impact: Directly attacks Availability

Go deeper:

• NIST CSRC glossary: denial of service — precise definition: prevention of authorized access or delay of time-critical operations.
• Denial-of-service attack (Wikipedia) — covers DoS vs DDoS (many sources), attack techniques, and defenses.