CWE-585: Empty Synchronized Block

What is Empty Synchronized Block?

• Overview: An Empty Synchronized Block is a coding issue where a synchronized block in Java contains no actual code. This means the block does not perform any synchronization and might indicate remnants of previous code changes that were not fully cleaned up.

• Exploitation Methods:

• Attackers can't directly exploit this vulnerability, as it typically doesn't introduce security risks on its own. However, it reflects poor code quality and maintenance practices, which could lead to more exploitable vulnerabilities elsewhere.
• Common attack patterns and techniques related to poor synchronization involve race conditions, but an empty block itself is not directly exploitable.

• Security Impact:

• Direct consequences of an empty synchronized block are generally limited to code readability and potential confusion for developers.
• Potential cascading effects include introducing misunderstandings in code logic, leading to incorrect assumptions about concurrency control.
• Business impact is minimal in terms of direct security threats but may contribute to increased maintenance costs and technical debt.

• Prevention Guidelines:

• Specific code-level fixes include removing the empty synchronized block entirely or ensuring that the block contains necessary code for synchronization.
• Security best practices involve regular code reviews to identify and clean up such issues, maintaining clear and concise code.
• Recommended tools and frameworks include static code analysis tools that can detect empty synchronized blocks and other similar issues.

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Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Java

Affected Technologies: Not specified

Vulnerable Code Example

// This Java class has a method with an empty synchronized block.
// Such blocks are misleading and unnecessary, and they can cause maintainability issues.
public class BankAccount {
    private int balance = 0;

    public void deposit(int amount) {
        synchronized (this) {  // An empty synchronized block provides no thread-safety benefits
            // No operations are performed here, making this block redundant
        }
    }
}

How to fix Empty Synchronized Block?

An empty synchronized block does not provide any thread-safety benefits and can be misleading to other developers, making them think that there's some critical section or shared resource access being protected when there isn't. To fix this issue, you should either remove the empty synchronized block or include the necessary operations that need synchronization within the block. If no operations require synchronization, the block should be removed entirely.

Fixed Code Example

// This fixed Java class now utilizes the synchronized block correctly 
// to ensure that the deposit operation is thread-safe.
public class BankAccount {
    private int balance = 0;

    public void deposit(int amount) {
        synchronized (this) {  // The synchronized block now correctly protects the critical section
            balance += amount;  // The balance update is a critical operation that needs synchronization
            System.out.println("Deposited " + amount + ", new balance: " + balance);  // Log the transaction
        }
    }
}

In the fixed code, the synchronized block now correctly protects the critical section where the balance is updated. This ensures that concurrent threads do not cause race conditions when modifying the account balance. This change improves both the functionality and clarity of the code, making it clear to other developers that the balance update is a critical operation requiring synchronization.