Slowloris is a type of denial-of-service (DoS) attack tool that targets web servers by opening multiple connections and sending partial HTTP requests to keep these connections open indefinitely. This method allows a single machine to overwhelm a server with minimal bandwidth, making it a highly efficient attack. The primary goal of Slowloris is to exhaust the server's resources, preventing it from handling legitimate requests.

How does Slowloris Work?

Slowloris operates by sending partial HTTP requests to a targeted web server, keeping these connections open for as long as possible. The attack begins by opening multiple connections to the server and sending incomplete HTTP headers. These partial requests are designed to appear legitimate, tricking the server into keeping the connections alive.

To maintain these connections, Slowloris periodically sends additional HTTP headers, ensuring the server does not time out the connections. This methodical approach allows the attacker to consume the server's resources gradually. As the server waits for the completion of these requests, it dedicates threads to each connection, eventually exhausting its capacity to handle new connections.

By continuously sending partial requests and keeping the connections open, Slowloris effectively monopolizes the server's resources. This prevents the server from processing legitimate requests, leading to a denial-of-service condition. The attack is particularly efficient because it requires minimal bandwidth from the attacker while causing significant disruption to the target server.

What are Examples of Slowloris Attacks?

Examples of Slowloris attacks include its use during the 2009 Iranian presidential election protests. Activists employed Slowloris to target government-run websites such as gerdab.ir, leader.ir, and president.ir. This strategic move aimed to disrupt the online presence of these sites without consuming significant bandwidth, thereby avoiding collateral damage to the internet access of the general population.

Another notable instance involved the spam network River City Media, which utilized a variant of Slowloris to force Gmail servers to send thousands of bulk messages. This attack demonstrated the potential for Slowloris to be exploited in email systems, causing significant operational disruptions and highlighting the need for robust security measures.

What are the Potential Risks of Slowloris?

The potential risks of suffering a Slowloris attack are significant and multifaceted. Here are some of the key risks:

• Resource Exhaustion: Slowloris can exhaust server resources by maintaining numerous open connections, leading to a complete denial-of-service.

• Prolonged Downtime: The attack can cause extended periods of downtime, as the server struggles to free up resources to handle new requests.

• Difficulty in Detection: Slowloris mimics legitimate traffic, making it challenging to identify and mitigate the attack promptly.

• Disruption of Legitimate User Access: Legitimate users may be unable to access services, leading to frustration and potential loss of business.

• Increased Operational Costs: Mitigating the attack often requires additional investments in hardware, software, and security measures, driving up operational costs.

How can you Protect Against Slowloris?

Protecting against Slowloris attacks requires a multi-faceted approach. Here are some effective strategies:

• Rate Limiting: Implement rate limiting to restrict the number of connections a single IP address can make, reducing the impact of malicious traffic.

• Timeout Settings: Configure your server to have aggressive timeout settings for incomplete HTTP requests, freeing up resources more quickly.

• Load Balancing: Use load balancers to distribute incoming traffic across multiple servers, preventing any single server from being overwhelmed.

• Web Server Configuration: Increase the maximum number of clients your web server can handle and limit the duration a client can stay connected.

• Reverse Proxies: Deploy reverse proxies to act as intermediaries, filtering and managing incoming traffic before it reaches your main server.