Need

The need is to implement measures that prevent or mitigate asymmetric denial of service attacks, ensuring the server remains responsive and available to legitimate users.

Context

Description

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.IOException;

public class VulnerableServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        if (param != null) {...

The above servlet `VulnerableServlet` is vulnerable to an asymmetric denial of service attack. It's a simple HTTP servlet that listens for GET requests. When it receives a request, it checks if the request contains a parameter named "param". If it does, it enters an infinite loop that writes a large number of responses back to the client. The vulnerability lies in the loop. By setting the loop to iterate `Integer.MAX_VALUE` times, it's possible to send a single request that will cause the server to generate an enormous number of responses. This is an example of an amplification attack, where a single request can consume a large amount of server resources. Additionally, if a malicious request is sent with a very large "param" value, it can cause the server to consume all of its available memory, causing it to become unresponsive. This is a type of denial of service attack. This code is vulnerable because it does not limit the number of responses it can generate from a single request, and it does not validate or limit the size of the "param" value. This allows an attacker to send a single request that can consume all of the server's resources, causing a denial of service.

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.IOException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import org.apache.commons.lang3.StringUtils;

...

The above code is a secure version of the original servlet. It implements rate limiting, input validation, and resource limits to prevent asymmetric denial of service attacks. 1. **Rate Limiting**: The `requestCount` variable is used to count the number of requests received per second. If the count exceeds `MAX_REQUESTS_PER_SECOND`, the server responds with a 429 (Too Many Requests) error. The count is reset every second by a background thread started in the `init` method. 2. **Input Validation**: The `param` request parameter is validated using the `StringUtils.isNumeric` method from Apache Commons Lang. If the parameter is not a valid number, the server responds with a 400 (Bad Request) error. 3. **Resource Limits**: The `param` request parameter is also used to limit the number of responses generated by the server. If the parameter value exceeds 1000, the server responds with a 400 (Bad Request) error. This prevents a single request from consuming an excessive amount of server resources. By implementing these measures, the server can effectively mitigate the risk of asymmetric denial of service attacks.

References