Denial-of-Service (DoS) attacks affect usual system operations by overloading them with unauthorized requests. These attacks directly affect available services, which may result in service interruptions, revenue loss, and reputational damage. In many cases, attackers exploit distributed networks and perform massive DoS attacks, which become hard to detect and defend. As digital infrastructure grows and becomes essential, the sophistication of DoS attacks increases. Security engineers should understand attack patterns, identify warning signs early, and implement effective defenses to protect systems and data. DoS attacks should be addressed using a combination of network hardening, traffic analysis, and continuous mitigation measures.

What are Denial-of-Service Attacks?

A Denial-of-Service (DoS) attack is a malicious attempt to stop a service, network, or application for actual users. Attackers overload a target system with traffic or exploit specific vulnerabilities that exhaust its resources. The purpose is to interrupt normal operations, reduce performance, or create complete service failures. DoS attacks can come from a single source or spread across a network of infected devices in the form of a Distributed Denial-of-Service (DDoS) attack. These attacks are a significant threat to organizations that rely on high availability and regular maintenance.

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How Does a DoS Attack Work?

A DoS attack helps to bring down an online service by overloading it with traffic from various sources. Here is how DoS works:

Botnet Formation

Attackers build or use botnets, which are networks of infected devices that they control. These devices are personal PCs or IoT technology and are infected with malware without the owner's awareness. Once they are affected, every device becomes a node in the attack network. The botnet allows the attacker to perform targeted attacks across thousands of endpoints. This distributed architecture increases the volume and the difficulty of traffic.

Traffic Flooding

The botnet results in massive amounts of fake traffic targeting the victim. Depending on the type of attack, this traffic can include HTTP requests, UDP packets, or ICMP echoes. The unexpected rise uses up bandwidth and server resources which causes the system to slow down or crash. Authorized users may face timeouts or service denials. If system is overloaded and exceeds capacity limits it makes application or service unavailable.

Exploiting Protocol Weaknesses

Some DoS attacks use the weakness of protocol vulnerabilities in TCP, DNS, or NTP. Attackers use inaccurate packets to exploit these vulnerabilities and use server resources. Protocol-based attacks demand less bandwidth but still cause significant risk. These attacks might be simple or difficult to detect without an extensive packet inspection.

Application-Layer Attacks

These attacks target Layer 7 of the OSI model, focusing on application logic rather than infrastructure.  They simulate normal user activity by sending legitimate HTTP queries, usually at a slow rate.  Despite the modest traffic volume, these requests cause the server to perform intensive operations like database searches.  The goal is to drain computer resources and reduce application performance.  Because the traffic appears genuine, traditional firewalls fail to block it.

Obfuscation and Diversion

To bypass detection, attackers often combine various types of traffic and attack channels. They also target various endpoints, switch protocols, or change request patterns. This unpredictability impacts filtering and finds rate restriction inefficient. In some cases, attackers use a low-volume attack to test defenses before launching an intensive attack. Diversion strategies also distract security engineers' attention away from more specific risks that are happening at the same time.

Types of Denial of Service Attacks

DoS attacks are of various types, and each targets a different level of a network or application stack. Here are the types of DoS attacks:

Volumetric Attacks

These attacks use all available bandwidth between the target and the internet. Attackers flood the network with massive amounts of traffic, like UDP overloads or DNS escalation.  They overload the connection, preventing legitimate users from accessing services.  These attacks rely on botnets to greatly increase the load.  However, security engineers can quickly detect them by monitoring significant increases in incoming bandwidth.

Protocol Attacks

Protocol attacks target network and transport protocols by exploiting vulnerabilities in the OSI model's Layers 3 and 4. For example, SYN overloads, fragmented packet attacks, and the Ping of Death. These attacks are specifically made to drain server resources like connection tables, CPU cycles, and memory buffers. Protocol attacks often use less bandwidth but can still cause system failures. They interrupt stateful resources like firewalls and load balancers.

Application Layer Attacks (Layer 7)

These attacks target the application layer, which handles HTTP, HTTPS, and DNS queries. Attackers replicate real user behavior to overload apps with requests that initiate resource-intensive procedures. Examples include HTTP floods and Slowloris attacks. These are more difficult to detect because they develop low traffic amounts that appear authentic. They are especially harmful to web services and APIs.

Amplification Attacks

Amplification attacks exploit weak third-party servers to replicate and increase traffic directed at a target. DNS, NTP, and Memcached are some of the most common vectors. Attackers submit low queries to these services using fake IPs, and they respond with huge responses to the target. This amplification ratio allows attackers to make significant changes with minimum effort. It also makes identifying the source more challenging.

Multi-Vector Attacks

Multi-vector attacks use two or more DoS methods in one campaign. An attacker may use a volumetric attack to overload bandwidth while also launching application-layer flooding. The idea is to confuse hackers and improve the chances of success. These attacks need significant coordination and are often used against targets of that has high value. Preventing this needs layered security and continuous traffic monitoring.

How to Identify a DoS Attack?

Security engineers identify DoS attacks by identifying changes in network behavior and resource performance. Here is how to identify DoS attacks

Unusual Traffic Patterns

Security engineers detect significantly increased traffic that differs from previous trends. Monitoring tools reveal an unexpected rise in packet traffic and connection requests. Alerts are triggered when traffic flows emerge from unexpected geographic regions. Unusual traffic distribution patterns indicate the use of a botnet.  Continuous inconsistencies in previously used bandwidth suggest that malicious operations are taking place.

Slow or Unresponsive Services

During an attack, systems may experience delays in processing valid user requests.  Under high-traffic loads, websites and applications slow down or stop working entirely.  It may also prevent error alerts and service interruptions.  Server response times may also slow down if resources are overloaded. Continuous delays in accessing services indicate poor network behavior.

Repeated Requests from Single IPs

Security engineers will find a huge amount of connection requests from specified IP addresses or IP ranges. The excessive repetition of requests overloads connection tables and service restrictions. This pattern suggests the existence of automated attack tools. The logs show persistent traffic from a limited number of sources. Concentrated request patterns help with isolating the risky components.

Overloaded Infrastructure Components

During a DDoS attack, network devices and servers use a huge amount of CPU and memory. Security engineers should check error logs to know about resource exhaustion and connection failures. System monitoring indicates an unexpected load on network interfaces and application servers, which may reduce the performance of firewalls and load balancers. When resource limits are surpassed, infrastructure components send alerts.

Alerts from Security Monitoring Tools

Automation and security monitoring tools help security engineers in the continuous detection of unusual traffic activity.   They receive alerts based on preset deviation metrics. These alerts often match with unusual increases in traffic. Monitoring dashboards will show how severe the attack is. Quick alerts will help fast incident response and mitigation actions.

How to Prevent and Mitigate DoS Attacks?

Security engineers will need a complete strategy to prevent and mitigate DoS attacks.

Implement Rate Limiting

Rate restriction limits various requests that a user or IP address can make and send within a given time range. It prevent exploitation by limiting excessive or automated traffic. Rate constraints reduce the efficiency of volumetric and application-layer attacks. It also saves server resources for legitimate users. Security engineers use rate limitations on APIs, login pages, and public endpoints.

Use Web Application Firewalls (WAF)

A WAF monitors incoming traffic and blocks malicious requests that target the application layer. It imposes rules for identifying known attack signatures and unusual behaviors.  WAFs prevent HTTP attacks, malware payloads, and request payloads.  They interact with content delivery networks (CDNs) to provide distributed security.  When properly configured, a WAF can prevent multiple attacks from reaching the source server.

Use Content Delivery Networks (CDNs)

CDNs distribute online content by globally scattered edge servers. They collect and balance incoming traffic, reducing pressure on the original server. CDNs prevent DoS threats by caching static content and detecting malicious requests. They also reduce latency and improve availability as traffic increases.  CDNs that are already connected with DoS protection offer more flexibility.

Source: Freepik