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What Is a Denial-of-Service (DoS) Attack?

Do you run a website, or online store or depend on services like banking or communication platforms? And if that is the case, think of them being offline all of a sudden and inaccessible to you and everyone else. This scenario is what a Denial-of-Service (DoS) attack looks like.

It is nothing but a malicious attempt to disrupt normal operations by overwhelming your online resources, essentially shutting down your digital presence and causing significant financial and reputational damage. Imagine thousands of fake requests flooding your servers, rendering legitimate users unable to access your services. 

Now, what exactly is a DoS attack, and how does it work? 

Continue reading to acquaint yourself with DoS attack techniques, motives, and the measures you can take to safeguard yourself and your valuable online existence.

Let’s jump right in!

What Is a Denial-of-Service (DoS) Attack?

What Is a Denial-of-Service (DoS) Attack?

A Denial-of-Service (DoS) attack is an attempt to flood the target website, service, or computer system with unwanted traffic, making it unavailable to its intended users. 

Let me simplify this. Suppose you have a restaurant that can only serve 100 customers at a time. A DoS attack is like someone sending thousands of fake orders at once, which makes your staff unable to handle the real orders, and prevents real customers from getting served. That is the simple essence of it.

In the digital world, a DoS attack intends to prevent legitimate users from accessing a website, online service, or computer. This is achieved by either flooding the target with traffic or exploiting vulnerabilities, hence jamming it and making it unable to function normally. It’s more like a digital traffic jam, blocking real users from their destination.

How Do DoS Attacks Work?

How Do DoS Attacks Work?

DoS attacks achieve their disruptive goals through two main methods: flooding and exploiting vulnerabilities. Let’s break them down:

1. Flooding:

Flooding the target with traffic: Imagine a highway suddenly swarmed with countless vehicles, causing a standstill. Similarly, DoS attacks bombard the target with overwhelming traffic, either:

Here’s how it works: Attackers send a huge number of requests to the target, either directly or through compromised devices. These requests can be:

  • Simple connection requests: Like constantly calling a busy phone line, these overwhelm the server’s ability to handle connections.

  • Large data requests: These overload the server’s bandwidth and processing power, leaving it unable to handle other tasks.

  • Amplification attacks: These exploit design flaws in other servers to amplify the attack traffic, creating a much larger impact.

2. Exploiting Vulnerabilities:

Exploiting hidden weakness: Attackers exploit bugs or flaws in the server’s software or security to crash it or consume its resources.

Here’s how it works: Attackers send specially crafted packets or data that trigger errors or unexpected behavior in the server. This can:

  • Crash the server: Causing a complete shutdown and denial of service.

  • Consume resources: Overwhelm the server’s CPU or memory, making it slow and unresponsive to legitimate requests.

Types of DoS Attacks

There are several types of DoS attacks, each with its characteristics and methods. Here are some common types:

UDP Flood: In this type of attack, the attacker overwhelms the target’s network by sending a large volume of User Datagram Protocol (UDP) packets. These packets are often sent to random ports, causing the target system to expend resources processing and responding to the fake requests.

TCP/IP Connection Exhaustion: Attackers exploit the finite nature of resources, such as the maximum number of concurrent connections a server can handle. By consuming all available connections, legitimate users are unable to establish new connections, leading to denial of service.

ICMP Flood: Internet Control Message Protocol (ICMP) floods involve overwhelming a target with ICMP echo request (ping) packets. The goal is to flood the network with a high volume of these requests, causing the target to become unreachable.

SYN/ACK Flood: This type of attack targets the TCP handshake process during the establishment of a connection. The attacker floods the target with many SYN (synchronize) or ACK (acknowledge) packets without completing the handshake, tying up resources and preventing legitimate connections.

Buffer Overflow: Exploits vulnerabilities in a program’s memory by overflowing a buffer with more data than it can handle. This excess data can overwrite adjacent memory, leading to the execution of malicious code or the manipulation of program behavior. Attackers often use this technique to inject and execute arbitrary code, potentially compromising the security of the system.

DNS Amplification: In a DNS amplification attack, the attacker exploits vulnerable DNS servers to amplify the volume of traffic directed at the target. By sending small DNS queries with a forged source IP address, the attacker tricks the DNS servers into sending larger responses to the victim.

HTTP/HTTPS Flood: Attackers flood a web server with a high volume of HTTP or HTTPS requests, overwhelming the server’s capacity to respond to legitimate user requests. This can lead to slow performance or complete unavailability of the web service.

Slowloris Attack: Slowloris is a low-and-slow type of attack that targets web servers by keeping numerous connections open and sending partial requests. This ties up the server’s resources as it waits for the incomplete requests to be completed, preventing it from serving legitimate requests.

Teardrop Attack: This involves sending a series of fragmented packets to a target system with overlapping offset fields. The intention is to confuse the reassembly process, leading to the system crashing or becoming unresponsive.

NTP Amplification: Similar to DNS amplification, this attack involves exploiting Network Time Protocol (NTP) servers to amplify the volume of traffic directed at the target.

Ping of Death: This older technique involves sending an oversized ICMP packet to a target. In the past, some systems were vulnerable to large ICMP packets, causing them to crash or become unresponsive.

Application Layer Attacks: These attacks target specific applications or services, aiming to exhaust resources such as database connections, bandwidth, or server processing power. Examples include SQL injection attacks and Cross-Site Scripting (XSS) attacks.

What is DDoS Attack, and how does it differ from DoS Attack?

DDoS attack is also a type of DoS attack but with a crucial distinction: the scale and distribution of the attack. It amplifies the impact by harnessing multiple compromised devices (often called a botnet) to launch a coordinated DoS attack from geographically dispersed locations.

Here’s what makes DDoS attacks particularly challenging:

  • Increased Attack Volume: The combined traffic from numerous compromised devices creates a much larger flood, exponentially amplifying the disruption.

  • Geographic Dispersion: Tracing the attack source becomes difficult due to the distributed nature of the botnet, making it harder to pinpoint and mitigate.

  • Redundancy and Obfuscation: Attackers can leverage different attack methods and constantly change their tactics, making it harder to predict and defend against.

How to investigate if under the DoS attack?

Identifying a DoS attack in real time can be crucial in minimizing its impact and initiating countermeasures. While specific symptoms might vary depending on the attack type, here are some key signs to watch out for:

Unusual Traffic Patterns:

  • Sudden Spikes: A drastic and unexplained increase in website traffic, particularly from unfamiliar locations or IP addresses, is a red flag. Monitor your analytics tools for anomalies.

  • Suspicious Traffic Patterns: Look for unusual patterns in traffic flows, like consistent peaks at odd hours or traffic originating from a single device type or geolocation.

  • Failed Login Attempts: A surge in failed login attempts could indicate a brute-force attack, potentially part of a broader DoS strategy.

Performance Issues:

  • Slow Loading Times: If your website or application experiences sluggishness, delays, or timeouts, it could be due to overwhelmed resources from excessive traffic.

  • Frequent Errors and Crashes: Unexplained errors, service outages, or unexpected server crashes might signal attempts to disrupt systems.

  • Limited Functionality: Certain features or functionalities becoming unavailable can point towards specific vulnerabilities being exploited.

Network and Server Issues:

  • High Resource Utilization: Monitor CPU, memory, and network bandwidth usage. Unexplained spikes exceeding typical levels could indicate malicious activity.

  • Connection Timeouts: If clients report trouble connecting or maintaining connections, it might be due to overloaded servers or network capacity exhaustion.

  • Security Alerts: Intrusion detection or security information & event management (SIEM) systems might raise alerts regarding suspicious activity or attempted DoS attacks.

How can you reduce the risk of a DoS attack?

Patch and Update Regularly: Keep your operating systems, applications, and firmware updated to counter the known vulnerabilities the hackers can exploit.

Monitor Traffic and Logs: Actively monitor network traffic and system logs for unusual spikes, suspicious patterns, or error messages that could indicate an attack. Cloud-based security information and event management (SIEM) solutions can help aggregate and analyze logs for easier detection.

Limit Attack Surface: Reduce the number of potential entry points for attackers. CloudDefense.AI’s CIEM solution helps identify and manage user access across your cloud environment, reducing the attack surface and potential vulnerabilities.

Input Validation and Sanitization: Implement robust input validation and sanitization techniques to prevent code injection attacks and other vulnerabilities.

Secure Authentication and Authorization: Use strong authentication protocols, enforce complex passwords, and implement least-privilege access control to limit potential damage if breached.

Regular Security Testing: Conduct regular testing and vulnerability assessments to identify and address weaknesses in your applications and infrastructure.

Stay Informed: Keep yourself updated on the latest DoS attack trends and techniques, and adapt your defenses accordingly.

Develop a Response Plan: Create a comprehensive incident response plan outlining steps to take when a DoS attack occurs, including communication protocols, mitigation strategies, and recovery procedures.


In our world today where everything is connected, DoS attacks pose serious threats for companies no matter their size. If you know how they work, see the early signs, and start using plans to prevent them ahead of time, you can cut down on the danger and damage these harmful efforts cause.

Always remember, it is most important to stop problems before they start. Make your network borders strong, ensure your servers and applications are secure, and always be watchful with monitoring and ready to respond quickly. Think about working together with security professionals such as CloudDefense.AI, who provide a full range of modern cloud security solutions to strengthen your protection and maintain continuous service when under attack. Book a FREE demo now and see it for yourself.

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Anshu Bansal
Anshu Bansal
Anshu Bansal, a Silicon Valley entrepreneur and venture capitalist, currently co-founds CloudDefense.AI, a cybersecurity solution with a mission to secure your business by rapidly identifying and removing critical risks in Applications and Infrastructure as Code. With a background in Amazon, Microsoft, and VMWare, they contributed to various software and security roles.
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