A Slowloris attack is a type of Denial of Service (DoS) attack that targets web servers by keeping many connections open and sending incomplete HTTP requests to them, preventing the server from closing the connections. This results in the server running out of resources as it waits for the rest of the request data that never arrives. The Slowloris attack is particularly effective against web servers like Apache, which use thread-based architectures to handle incoming requests.
The Slowloris attack exploits the way web servers handle incoming HTTP requests. When a client, such as a web browser, makes a request to a server, it sends the full HTTP request and expects a response. However, in a Slowloris attack, the attacker sends a partial HTTP request and holds the connection open by sending headers very slowly. Here’s how it works:
Send Partial HTTP Requests: The attacker sends the beginning of an HTTP request (such as the GET method) along with the headers. The request is not completed, leaving the server expecting additional data.
Keep Connections Open: To maintain the connection, the attacker periodically sends more small, incomplete parts of the HTTP request, preventing the server from closing the connection.
Exhaust Server Resources: The server, waiting for the rest of the request to be completed, keeps the connection open. If the attacker sends enough incomplete requests, the server will exhaust its available threads or processes, causing it to slow down or stop responding to legitimate users.
The primary advantage of a Slowloris attack is its ability to cause significant service disruption with minimal traffic and relatively low resource consumption from the attacker. The benefits of understanding and mitigating Slowloris include:
Efficient Resource Consumption: Unlike traditional DDoS attacks, which generate massive amounts of traffic, the Slowloris attack can overwhelm a server with very little data, making it more difficult to detect and mitigate.
Simple and Stealthy: Slowloris is relatively simple to execute and can be conducted from a single machine. It’s also stealthy, as it doesn’t generate a large traffic volume, making it hard to distinguish from legitimate traffic.
Targeting Specific Web Servers: The attack is most effective against certain types of web servers (e.g., Apache). If a business uses such a server configuration, awareness of Slowloris can help implement proactive protections.
Despite its effectiveness, there are several challenges and considerations associated with defending against Slowloris attacks:
Detection Difficulty: Since Slowloris doesn’t generate an unusually high volume of traffic, it can be difficult to distinguish from legitimate traffic. Administrators may not notice the attack until the server starts to fail.
Impact on Server Performance: Even a small number of attackers can cause a significant slowdown or denial of service. Web servers that use thread-based models (like Apache) are especially vulnerable because the attacker consumes server resources without sending actual data.
Limited Effectiveness on Modern Systems: Many modern servers have implemented defenses that make Slowloris attacks less effective. Servers may limit the amount of time a connection can stay open or automatically close connections if they detect suspicious behavior.
To protect against Slowloris attacks, several strategies can be implemented:
Configure Timeouts and Connection Limits: Setting appropriate timeouts and connection limits on the server can help reduce the likelihood of an attack succeeding. For example, a server could be configured to close idle connections after a certain period.
Use Reverse Proxies: Deploying a reverse proxy server, such as Nginx or HAProxy, in front of your web server can help detect and block Slowloris attacks. These proxies can handle incoming requests and pass legitimate traffic to the web server while filtering out incomplete or suspicious requests.
Rate Limiting: Rate limiting can be configured to restrict the number of incoming connections from a single IP address, helping to reduce the number of slow connections a malicious actor can establish.
Intrusion Detection Systems (IDS): An IDS can detect unusual patterns in network traffic, such as incomplete HTTP requests or repeated connection attempts, and raise an alert before the server is overwhelmed.
Web Application Firewalls (WAF): A WAF can help filter and block traffic that appears to be part of a Slowloris attack. For example, WAFs can be configured to drop connections that lack the proper headers or are incomplete.
Use of Non-threaded Servers: Consider using event-driven or non-threaded web servers (e.g., Nginx, LiteSpeed) which do not rely on individual threads for each request. These servers are more resilient to Slowloris attacks, as they handle many connections more efficiently.
While Slowloris attacks are less common than some other DDoS methods, they remain an effective tool for attackers targeting specific vulnerabilities in web server configurations. By understanding how the attack works and implementing appropriate mitigations, organizations can ensure their systems remain resilient to this form of service disruption.