CWE-567: Unsynchronized Access to Shared Data in a Multithreaded Context

What is Unsynchronized Access to Shared Data in a Multithreaded Context?

• Overview: This vulnerability occurs when shared data in a multithreaded environment is accessed without proper synchronization, leading to unpredictable behavior and data corruption.

• Exploitation Methods:

• Attackers can exploit this by manipulating thread execution to cause data inconsistencies.
• Common techniques include race conditions where multiple threads access and modify shared data simultaneously.

• Security Impact:

• Direct consequences include data corruption, inconsistent application state, and application crashes.
• Potential cascading effects involve further security issues like unauthorized data access.
• Business impact could be severe, leading to loss of data integrity, reduced reliability, and compromised user trust.

• Prevention Guidelines:

• Use synchronization mechanisms such as locks, semaphores, or synchronized blocks to control access to shared data.
• Follow security best practices like avoiding the use of shared mutable state whenever possible.
• Recommended tools and frameworks include Java's concurrent package utilities like ReentrantLock, ReadWriteLock, and ConcurrentHashMap.

Corgea can automatically detect and fix Unsynchronized Access to Shared Data in a Multithreaded Context in your codebase. [Try Corgea free today](https://corgea.app).

Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Java

Affected Technologies: Not specified

Vulnerable Code Example

Java Example

public class Counter {
    private static int count = 0; // Shared static variable

    public static void increment() {
        count++; // Unsynchronized access to shared data
    }

    public static int getCount() {
        return count; // Unsynchronized access to shared data
    }
}

public class CounterTest {
    public static void main(String[] args) {
        Thread t1 = new Thread(Counter::increment);
        Thread t2 = new Thread(Counter::increment);

        t1.start();
        t2.start();

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Final Count: " + Counter.getCount());
    }
}

Vulnerability Explanation

• The Counter class uses a static variable count which is accessed by multiple threads without synchronization.
• This can lead to a race condition where multiple threads update the count variable simultaneously, resulting in incorrect final values.
• For example, if two threads read the same value of count before either writes back, both may perform the same increment operation, leading to a lost update.

How to fix Unsynchronized Access to Shared Data in a Multithreaded Context?

To fix this issue, we need to ensure that access to the shared data (count) is synchronized. This can be achieved using several approaches:

1. Synchronized Methods: Make the methods that access the shared data synchronized.
2. Synchronized Blocks: Use synchronized blocks with an appropriate lock to protect the access to shared data.
3. Atomic Variables: Use atomic classes from java.util.concurrent.atomic package, like AtomicInteger, to provide thread-safe operations on integers.

Best Practice

Using the AtomicInteger class is often preferred for counters, as it provides atomic operations out of the box, which are more efficient than using synchronized methods or blocks.

Fixed Code Example

import java.util.concurrent.atomic.AtomicInteger;

public class Counter {
    private static final AtomicInteger count = new AtomicInteger(0); // Use AtomicInteger for thread-safe operations

    public static void increment() {
        count.incrementAndGet(); // Atomic operation ensures thread safety
    }

    public static int getCount() {
        return count.get(); // Atomic read operation
    }
}

public class CounterTest {
    public static void main(String[] args) {
        Thread t1 = new Thread(Counter::increment);
        Thread t2 = new Thread(Counter::increment);

        t1.start();
        t2.start();

        try {
            t1.join();
            t2.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("Final Count: " + Counter.getCount());
    }
}

Fix Explanation

• The count variable is now an AtomicInteger, which provides methods like incrementAndGet() and get() that are atomic and thread-safe.
• This eliminates the need for explicit synchronization, as AtomicInteger handles it internally, making the code both simpler and more efficient.
• By using AtomicInteger, we avoid potential race conditions and ensure that increments are performed safely even when accessed by multiple threads concurrently.