阻断服务攻击（英语：Denial of Service Attack，缩写：DoS）亦称洪水攻击，是一种网络攻击手法，其目的在于使目标电脑的网络或系统资源耗尽，使服务暂时中断或停止，导致其对目标客户不可用。

当骇客使用网络上两个或以上被攻陷的电脑作为「僵尸」向特定的目标发动「拒绝服务」式攻击时，其称为分布式阻断服务攻击（Distributed Denial of Service attack，缩写：DDoS）。据2014年统计，被确认为大规模DDoS的攻击已达平均每小时28次。攻击发起者一般针对重要服务进行攻击，如银行，信用卡支付网关，甚至根域名服务器。

In computing, a denial-of-service (DoS) attack is an attempt to make a machine or network resource unavailable to its intended users, such as to temporarily or indefinitely interrupt or suspend services of a host connected to the Internet. A distributed denial-of-service (DDoS) is where the attack source is more than one, often thousands of, unique IP addresses. It is analogous to a group of people crowding the entry door or gate to a shop or business, and not letting legitimate parties enter into the shop or business, disrupting normal operations.