What Is the Slowloris Attack? How Does a Layer 7 DDoS Using Partial HTTP Requests Work? Explained with Other Related Attack Types

Table of Contents

• What Is a Layer 7 DDoS Attack?
• Which Attack Is Described?
• What is the Slowloris Attack?
• Other Attack Options Explained
•  Comparing Layer 7 DDoS Attacks
• How to Defend Against Slowloris and Similar Layer 7 Attacks
• Conclusion
• Frequently Asked Questions (FAQs)

In the evolving landscape of cybersecurity threats, Distributed Denial of Service (DDoS) attacks continue to be a significant concern for organizations relying on web infrastructure. Among the various types of DDoS attacks, those targeting Layer 7 (Application Layer) are especially dangerous because they exploit the way web servers handle HTTP requests, leading to resource exhaustion and service downtime.

This blog focuses on a specific Layer 7 DDoS attack where partial HTTP requests are sent to the web server, causing it to hold multiple connections open and wait indefinitely for request completion. We will explore the Slowloris attack, explain its mechanics, compare it with similar attack types, and discuss ways to defend against it.

What Is a Layer 7 DDoS Attack?

Before diving into the Slowloris attack, it’s essential to understand Layer 7 DDoS attacks. These attacks target the Application Layer of the OSI model, specifically focusing on protocols like HTTP, HTTPS, DNS, and SMTP.

Unlike volumetric DDoS attacks that overwhelm bandwidth, Layer 7 attacks aim to exhaust server resources by sending malicious requests that appear legitimate but are designed to consume CPU, memory, or connection pool limits, disrupting services without requiring massive traffic.

Which Attack Is Described?

"Partial HTTP requests are sent to the web infrastructure or applications. Upon receiving a partial request, the target servers open multiple connections and keep waiting for the requests to complete."

The attack described is known as the Slowloris attack.

What is the Slowloris Attack?

How Slowloris Works

The Slowloris attack is a type of Layer 7 DDoS attack that targets web servers by opening multiple HTTP connections and keeping them alive as long as possible by sending partial or incomplete HTTP requests at a very slow rate.

Here’s the step-by-step process:

1. The attacker sends a partial HTTP request header to the server but deliberately never completes it.

2. The server opens a connection and keeps it alive, waiting for the rest of the HTTP headers.

3. The attacker continuously sends small header segments to keep the connection alive but never completes the request.

4. Over time, the server’s connection pool becomes exhausted, unable to accept new legitimate requests.

5. This leads to denial of service, where genuine users cannot access the web application.

Why Slowloris Is Dangerous

• Stealthy: Slowloris uses minimal bandwidth, making it difficult to detect by traditional volumetric DDoS defenses.

• Effective: Can bring down large web servers by exhausting their connection handling capabilities.

• Targets Specific Servers: Works best on web servers like Apache, which allocate resources per connection.

Other Attack Options Explained

Let’s briefly discuss the other attack options mentioned for clarity:

Session Splicing

• Involves splitting HTTP requests or responses into multiple packets to evade detection by intrusion prevention systems.

• Not primarily a DDoS method but used in evasion techniques.

Phlashing (Permanent DoS)

• Targets hardware devices like routers or firewalls by sending malformed packets causing hardware to crash.

• Causes permanent damage, requiring manual reboot or replacement.

Desynchronization Attack

• Also known as HTTP request smuggling.

• Exploits discrepancies in how front-end and back-end servers parse requests to bypass security or cause misrouting.

• Not focused on resource exhaustion but on injecting malicious requests.

 Comparing Layer 7 DDoS Attacks

How to Defend Against Slowloris and Similar Layer 7 Attacks

1. Configure Server Timeout Settings

• Set reasonable timeouts for HTTP headers to prevent connections from staying open indefinitely.

• Limit the maximum number of simultaneous connections per IP.

2. Use Web Application Firewalls (WAF)

• WAFs can detect abnormal request patterns and block suspicious traffic, including slow HTTP attacks.

3. Employ Reverse Proxies and Load Balancers

• Reverse proxies can limit connections and filter malicious requests before they reach backend servers.

• Load balancers distribute traffic efficiently to avoid resource exhaustion.

4. Monitor Traffic Patterns

• Set up anomaly detection to identify low bandwidth but high connection count attacks.

• Use tools to monitor active connections and their duration.

5. Disable Unnecessary Features

• Disable or restrict modules that allow long-held HTTP connections if not required.

Conclusion

The Slowloris attack is a clever and stealthy Layer 7 DDoS technique that exploits the way web servers handle partial HTTP requests to exhaust connection pools and bring down web infrastructure. Unlike traditional high-bandwidth DDoS attacks, Slowloris uses minimal traffic, making detection and mitigation challenging.

By understanding how Slowloris and other related attacks like session splicing, phlashing, and desynchronization work, security professionals can better defend their web applications with proper server configurations, firewalls, and traffic monitoring.

If you're interested in learning hands-on techniques to detect, analyze, and mitigate such attacks, enrolling in cybersecurity and ethical hacking courses will equip you with the practical skills needed to protect modern web infrastructures.

FAQs 

What is a Layer 7 DDoS attack?

A Layer 7 DDoS attack targets the Application Layer by exhausting server resources through malicious HTTP or other protocol requests.

What is the Slowloris attack?

Slowloris is a Layer 7 DDoS attack where attackers send partial HTTP requests to keep many connections open, exhausting server connection pools.

How does Slowloris differ from other DDoS attacks?

Unlike volumetric attacks, Slowloris uses low bandwidth but holds server connections open for long durations.

What types of web servers are vulnerable to Slowloris?

Web servers like Apache and older IIS versions that allocate resources per connection are most vulnerable.

What is session splicing in cybersecurity?

Session splicing involves splitting HTTP requests into multiple packets to evade detection by security systems.

What is phlashing?

Phlashing is a permanent DoS attack that targets hardware devices causing permanent damage or crashes.

How does a desynchronization attack work?

It exploits differences in HTTP request parsing between servers to bypass security or cause misrouting.

Can Slowloris attacks be detected easily?

No, Slowloris is stealthy because it uses minimal traffic, making traditional volumetric detection ineffective.

What are effective defenses against Slowloris?

Setting connection timeouts, limiting concurrent connections, using WAFs, reverse proxies, and monitoring traffic patterns.

Why is configuring server timeouts important?

It prevents malicious clients from holding server connections indefinitely, reducing attack impact.

What role do web application firewalls play in DDoS protection?

WAFs filter malicious HTTP requests and block abnormal traffic patterns indicative of Layer 7 attacks.

Can load balancers help mitigate Slowloris attacks?

Yes, load balancers distribute traffic and can limit connections per client, helping prevent resource exhaustion.

Is Slowloris attack bandwidth intensive?

No, it requires very low bandwidth but exploits server connection handling.

Are modern web servers vulnerable to Slowloris?

Many modern servers have protections, but some configurations can still be vulnerable if not hardened.

What are partial HTTP requests in the context of Slowloris?

Requests where headers are sent slowly or incompletely to keep connections open without completion.

How can developers test if their server is vulnerable to Slowloris?

By using penetration testing tools that simulate Slowloris traffic and monitor server response.

Is phlashing a common attack today?

It is rare but very damaging since it can cause hardware failures.

Can session splicing lead to DDoS attacks?

Session splicing is mostly an evasion technique rather than a direct DoS method.

How does desynchronization relate to HTTP request smuggling?

It’s a form of HTTP request smuggling exploiting parsing discrepancies.

Does using HTTPS protect against Slowloris?

HTTPS encrypts traffic but does not prevent resource exhaustion attacks like Slowloris.

What tools help detect Slowloris attacks?

Network monitors, anomaly detection systems, and specific security tools like Fail2Ban can help.

Is Slowloris used in combination with other attacks?

Yes, attackers may combine Slowloris with volumetric or other Layer 7 attacks for amplification.

Can cloud-based DDoS protection services mitigate Slowloris?

Many cloud services offer protections, but effectiveness varies depending on configuration.

What is the impact of Slowloris on business operations?

It can cause website downtime, lost revenue, and damage to reputation.

Are IoT devices susceptible to Slowloris?

IoT devices running HTTP services can be vulnerable if poorly configured.

How often should organizations test for Layer 7 vulnerabilities?

Regularly, especially after deploying new applications or infrastructure changes.

What learning resources cover Slowloris and Layer 7 attacks?

Courses on ethical hacking, penetration testing, and web security provide practical knowledge.

Can AI-based security tools detect Slowloris attacks?

Yes, AI tools analyzing traffic behavior can improve detection rates.

What is the first step in mitigating a Slowloris attack?

Identifying abnormal connection patterns and adjusting server configurations to limit them.

How important is logging for detecting Layer 7 attacks?

Extremely important; detailed logs help identify slow or incomplete requests indicative of attacks.