What does DoS stand for?
DoS stands for Denial of Service. An attack that makes a service unavailable by exhausting its bandwidth, connections or resources, or by crashing it.
A DoS attack stops legitimate users from reaching a system or service. The attacker either floods the target with traffic or requests until its bandwidth, memory or connection table is exhausted, or sends input that makes the software crash or hang. The security property under attack is availability: nothing is stolen or changed, but the service is down. When the attack comes from many machines at once it is called distributed.
DoS in practice: an example
A single machine opens thousands of connections to a web server and deliberately sends each request very slowly, a few bytes at a time. The server keeps every connection open while it waits, runs out of worker threads and stops answering real visitors. Blocking that one source address ends the attack.
What DoS is confused with
DoS vs DDoS: what is the difference?
A DoS attack has one source. A DDoS attack uses many sources at the same time, which makes it larger and much harder to block by address. What DDoS stands for
Which exams cover DoS?
DoS is on the official CompTIA acronym list for two of the four exams covered here.
- Security+ SY0-801: 2.5 (Threats, Vulnerabilities, and Attacks).
- A+ 220-1201 and 220-1202: 220-1202 2.0 (Security).
Each number is a CompTIA exam objective and the name in brackets is its domain. The exam puts the term in a scenario; it does not ask you to recite the expansion.
Check yourself: two practice questions
Written by CertSprint for this page. They are not taken from any exam.
1. Which part of the confidentiality, integrity and availability triad does a DoS attack target?
- Confidentiality
- Integrity
- Availability
- Non-repudiation
Show the answer
C. Availability. The aim is to make the service unreachable. No data has to be read or altered for the attack to succeed.
2. A single workstation sends malformed packets that crash a router every few minutes. What is this?
- A DoS attack
- A brute-force attack
- Tailgating
- Data exfiltration
Show the answer
A. A DoS attack. One source making a device unavailable is a denial of service. It would be distributed only if many sources took part.
Sources: NIST glossary: denial of service; RFC 4732: Internet Denial-of-Service Considerations. The acronym and its expansion come from CompTIA's official acronym lists, linked from each exam glossary above. The explanation, example and questions are CertSprint's own, checked against these sources on 11 November 2026.