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.
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 {
String param = request.getParameter("param");
if (param != null) {
for (int i = 0; i < Integer.MAX_VALUE; i++) {
response.getWriter().write("Response " + i + "\\n");
}
}
}
}
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;
public class SecureServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
private static final int MAX_REQUESTS_PER_SECOND = 5;
private AtomicInteger requestCount = new AtomicInteger(0);
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
if (requestCount.incrementAndGet() > MAX_REQUESTS_PER_SECOND) {
response.sendError(HttpServletResponse.SC_TOO_MANY_REQUESTS, "Too many requests");
return;
}
String param = request.getParameter("param");
if (StringUtils.isNumeric(param)) {
int limit = Integer.parseInt(param);
if (limit > 1000) {
response.sendError(HttpServletResponse.SC_BAD_REQUEST, "Request limit exceeded");
return;
}
for (int i = 0; i < limit; i++) {
response.getWriter().write("Response " + i + "\\n");
}
} else {
response.sendError(HttpServletResponse.SC_BAD_REQUEST, "Invalid request");
}
requestCount.decrementAndGet();
}
@Override
public void init() throws ServletException {
super.init();
new Thread(() -> {
while (true) {
requestCount.set(0);
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}).start();
}
}
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.