Application Layer DDoS Attack – What it is, Types & Mitigation

What is an Application Layer DDoS Attack?

An application layer DDoS attack, also known as a Layer 7 (L7) DDoS attack, targets the application layer of the OSI model. This type of DDoS attack focuses on disrupting specific functions or features of a website or online service.

Layer 7 attacks leverage loopholes, vulnerabilities, or business logic flaws in the application layer to orchestrate the attacks.

Here are the key characteristics and methods:

• Targeted at the Application Layer: Unlike network-layer attacks (e.g., SYN flood or UDP flood) that focus on overwhelming the network infrastructure, layer 7 DDoS attacks target the application itself.
• Low Bandwidth Consumption: These attacks require less bandwidth to bring down a site compared to network-layer attacks because they exploit application vulnerabilities and resource-intensive processes.
• Sophisticated and Hard to Detect: Attackers send seemingly legitimate requests to take down the application; often requesting access to load a single page. These very qualities make OSI Layer 7 attacks much sneakier and more dangerous.

Examples of L7 attacks are Slowloris, GET/POST Floods, etc.

How Do Application Layer DDoS Attacks Work?

The OSI layer 7 DDoS attack works by overwhelming the application layer of a target system with a flood of malicious requests that mimic legitimate user behavior.

The attack typically starts with reconnaissance, where attackers select a target application, such as a website or online service, and identify vulnerabilities or resource-intensive operations that can be exploited. Next, they recruit a botnet, a network of compromised devices infected with malware and controlled remotely, to scale the attack.

Once the botnet is ready, the attacker sends commands to initiate the attack. The DDoS botnet then sends a high volume of HTTP GET or POST requests, often targeting resource-intensive operations like database queries or large file downloads.

Other techniques include Slowloris attacks, which open multiple connections to the web server but send data very slowly, and DNS query floods that target DNS servers with a high volume of queries for non-existent domains. These actions aim to exhaust the server’s resources, such as CPU, memory, and bandwidth.

As the volume of malicious requests increases, the server becomes overloaded, leading to service degradation. Legitimate users experience slow response times or are unable to access the service altogether.

Eventually, the application may crash or become completely unresponsive, achieving the attacker’s goal of denying service.

What are the Types of Layer 7 DDoS Attacks?

L7 DDoS attacks are particularly challenging to detect because they imitate legitimate user behavior, making distinguishing between normal traffic and malicious activity difficult. There are several forms these attacks can take, including:

1. HTTP Floods: Overwhelming the server with a high volume of HTTP GET or POST requests, often targets resource-intensive pages or functionalities. HTTP Flooding is the most common application-layer DDoS attack and can be categorized into four different types:

• Basic HTTP Floods: These are the simplest and most common types of HTTP Flooding attacks. Attackers use a limited range of IP addresses, user agents, and referrers to repeatedly access the same webpage or resource. The server, unable to handle the sudden influx of requests, eventually crashes.
• Randomized HTTP Floods: In these more complex attacks, attackers use a wide range of IP addresses, randomized URLs, user agents, and referrers. Botnets, consisting of numerous devices infected with malware, send GET/POST requests to the server. This randomization makes the attack harder to detect and mitigate.
• Cache-Bypass HTTP Floods: A sub-category of Randomized HTTP Flooding, these L7 attacks involve strategies to bypass web application caching systems. Attackers search for un-cached content or use generic dictionary searches, forcing the server to consume significant bandwidth and resources, leading to downtimes. Cache-bypass flooding attacks are considered among the most sophisticated.
• WordPress XML-RPC Floods: Attackers leverage the simple XML-RPC pingbacks from multiple WordPress installations as a reflection mechanism to orchestrate the flooding attack.

2. Slowloris Attack

Another notable application-layer DDoS attack is the Slowloris Attack. This attack is both simple and highly effective. Unlike volumetric attacks that bombard the server with numerous requests, Slowloris sends payloads very slowly, keeping the connection open for a long period.

Even at low volumes, these slow payloads can exhaust the server’s connection pool, preventing it from serving legitimate users. This method can be devastating despite its simplicity, making it one of the most lethal application-layer DDoS attacks.

3. DNS Query Floods

DNS Floods involve sending a large volume of DNS requests to a DNS server, often for non-existent domains. These requests cannot be cached, forcing the DNS server to expend significant resources attempting to resolve them. The high volume of requests, often originating from a botnet, aims to overwhelm the server, leading to slow response times or complete unavailability for legitimate users.

This distributed nature of the attack makes it difficult to identify and block malicious traffic, potentially impacting the entire DNS infrastructure and causing broader service disruptions.

4. Resource-Intensive Queries

Resource-intensive queries exploit application features that require significant processing power, such as complex database queries or file uploads. Attackers craft complex SQL queries that consume considerable server resources, leading to degraded performance or outages.

These attacks target the application’s backend, straining databases and file systems, and ultimately disrupting the normal operation of the service.

These attacks are designed to blend in with regular application use, making them harder to detect and mitigate compared to other types of DDoS attacks.

How Can You Mitigate Application Layer DDoS Attacks?

To effectively mitigate Layer 7 DDoS attacks, several techniques and tools can be implemented:

1. Implement Rate Limiting and Throttling

Control the number of requests per second from individual IP addresses or specific URLs to prevent overwhelming the server with high-volume traffic, such as HTTP floods.

2. Utilize IP Reputation Filtering

Block traffic from known malicious IP addresses or botnets using IP reputation lists to mitigate attacks originating from sources with a history of malicious behavior.

3. Integrate CAPTCHAs and Behavioral Challenges

Add CAPTCHAs or behavioral challenges on critical web pages to distinguish human users from automated bots, enhancing security against automated scripts.

4. Inspect HTTP Headers and Payloads

Proactively inspect HTTP headers and payloads for anomalies or malicious content, such as SQL injection attempts or malformed requests, to detect and block application layer attacks.

5. Offload and Inspect SSL/TLS

Offload SSL/TLS decryption to a dedicated device or service to inspect encrypted traffic, enabling deep packet inspection to detect and mitigate hidden attacks within HTTPS requests.

6. Establish Real-Time Monitoring and Incident Response

Continuously monitor application traffic and system logs to detect unusual patterns or spikes indicative of DDoS attacks. Implement prompt incident response actions to minimize attack impact and duration.

7. Conduct Regular Security Audits and Updates

Perform frequent security audits of web applications and WAF configurations to identify and remediate vulnerabilities, ensuring defenses remain effective against evolving threats.

8. Deploy a Web Application Firewall (WAF)

Use a WAF to shield applications from various DDoS attacks. WAF can manage, filter, and analyze traffic from different sources. Managed WAFs screen the layer 7 traffic and feed data directly to cybersecurity experts who can analyse the behavior of malicious traffic trying to disrupt your services. Know more about how a WAF works here.

9. Leverage DDoS Mitigation Software

However, most DDoS mitigation solutions primarily focus on volumetric attacks and fail to provide comprehensive security against layer 7 attacks.

Choose DDoS protection software that offers comprehensive security against all types of DDoS attacks. Ensure the solution provides instant protection, real-time alerts, custom rules, security analytics, and access to cybersecurity experts

For more detailed insights on how to stop DDoS attacks, check out our blog post How to Stop DDoS Attack.

How Does AppTrana Help Mitigate Layer 7 DDoS Attacks?

The AppTrana WAAP includes a fully managed behavioral-based DDoS mitigation solution designed to rapidly defend against layer 3-7 DDoS attacks. Unlike traditional methods with static limits, AppTrana’s policies adapt dynamically based on observed application request behaviors.

Using AI and machine learning, AppTrana monitors and learns from user and entity behaviors, continuously analyzing traffic patterns to detect anomalies. This approach leverages advanced data analysis, log insights, and threat intelligence to identify potentially malicious activities. Key features include Good Bot Pretender Detection, Fingerprinting and JavaScript Detection, Integrity Checks, and Behavior Anomaly Detection, collectively ensuring robust protection against malicious bots orchestrating the attacks.

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