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Race condition

Need​

Ensuring proper synchronization and ordering of inputs to prevent race conditions

Context​

  • Usage of Java 8 for developing applications with enhanced features and performance
  • Usage of javax.servlet-api for building Java web applications using the Servlet API

Description​

Non compliant code​

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class RaceConditionServlet extends HttpServlet {

    private int counter = 0;

    protected void doGet(HttpServletRequest request, HttpServletResponse response) {
        counter++;
        String param = request.getParameter("param");
        if ("reset".equals(param)) {
            counter = 0;
        }
        response.getWriter().println("Counter: " + counter);
    }
}

This code represents a servlet that increases a counter each time the doGet method is called. If the parameter "param" of the request is equal to "reset", the counter is reset to 0. The counter value is then written to the response.

The vulnerability lies in the fact that the doGet method is not thread-safe. Servlets are typically multithreaded in Java, meaning that multiple threads can execute the doGet method at the same time. This can lead to a race condition.

For example, if one thread is in the process of incrementing the counter, and another thread comes in and resets the counter before the first thread has finished, the counter might not be incremented properly. This is because the increment operation is not atomic and consists of three separate operations: reading the current value, incrementing it, and writing the new value back. If the counter is reset during this sequence, the increment is lost.

Therefore, this code is vulnerable to race conditions, which can lead to unpredictable behavior and incorrect results.

Steps​

  • Use thread synchronization mechanisms to ensure atomicity of the counter increment and reset operations.
  • Implement a locking mechanism to prevent multiple threads from accessing and modifying the counter simultaneously.
  • Consider using a thread-safe data structure, such as AtomicInteger, to handle the counter variable.
  • Ensure that the order of the inputs does not affect the functionality by implementing proper validation and handling of the input parameters.

Compliant code​

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.util.concurrent.atomic.AtomicInteger;

public class RaceConditionServlet extends HttpServlet {

    private AtomicInteger counter = new AtomicInteger(0);

    protected synchronized void doGet(HttpServletRequest request, HttpServletResponse response) {
        counter.incrementAndGet();
        String param = request.getParameter("param");
        if ("reset".equals(param)) {
            counter.set(0);
        }
        response.getWriter().println("Counter: " + counter);
    }
}

The code above is a revised version of the original servlet that had a race condition vulnerability. The vulnerability was due to the non-atomic increment and reset operations on the counter variable, which could lead to unexpected behavior if multiple threads accessed and modified the counter simultaneously.

To fix this vulnerability, several changes were made:

  1. AtomicInteger: The counter variable was changed from an int to an AtomicInteger. AtomicInteger is a thread-safe class in Java that provides methods for atomically incrementing, decrementing, and setting the value of the integer. This ensures that the increment and reset operations on the counter are atomic, preventing race conditions.

  2. Synchronized Method: The doGet method was made synchronized. This means that only one thread can access this method at a time. If another thread tries to access this method while it is being used by another thread, it will have to wait until the first thread is done. This prevents multiple threads from accessing and modifying the counter simultaneously, which was the cause of the race condition.

  3. Proper Validation and Handling of Input Parameters: The code checks if the input parameter is "reset" and only then resets the counter. This ensures that the order of the inputs does not affect the functionality.

By making these changes, the race condition vulnerability in the servlet has been fixed.

References​

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