What is Empty Synchronized Block (CWE-585)?

TL;DR: Empty Synchronized Block (CWE-585) is a Java security issue where an empty synchronized block waits for synchronization without performing actions, leading to potential race conditions. Fix by ensuring synchronized blocks contain necessary logic or removing them.

What is Empty Synchronized Block?

Empty Synchronized Block (CWE-585) is a type of Java security vulnerability where an empty synchronized block waits for synchronization without performing any actions. As defined by the MITRE Corporation under CWE-585, and classified by the OWASP Foundation as not directly mapped to their Top 10:2025.

Quick Summary

Empty Synchronized Block is a Java-specific issue that can lead to race conditions when other threads modify shared resources during synchronization waits. It matters because it undermines thread safety and can cause data integrity issues in multi-threaded applications. Jump to: Overview · How It Works · Business Impact · Attack Scenario · Detection · Fixes

Jump to: Quick Summary · Empty Synchronized Block Overview · How Empty Synchronized Block Works · Business Impact of Empty Synchronized Block · Empty Synchronized Block Attack Scenario · How to Detect Empty Synchronized Block · How to Fix Empty Synchronized Block · Framework-Specific Fixes for Empty Synchronized Block · How to Ask AI to Check Your Code for Empty Synchronized Block · Empty Synchronized Block Best Practices Checklist · Empty Synchronized Block FAQ · Vulnerabilities Related to Empty Synchronized Block · References · Scan Your Own Site

Empty Synchronized Block Overview

What

Empty Synchronized Block is a Java-specific vulnerability where an empty synchronized block waits for synchronization without performing any actions.

Why it matters

It can lead to race conditions, allowing other threads to modify shared resources during the wait period, impacting system integrity and availability.

Where it occurs

In multi-threaded applications that use synchronized blocks but do not utilize them properly.

Who is affected

Developers using Java who rely on synchronization mechanisms without proper implementation.

Who is NOT affected

Applications that do not use or misuse empty synchronized blocks in their codebase.

How Empty Synchronized Block Works

Root Cause

The root cause of this issue lies in the improper usage of synchronized blocks, where an empty block waits for synchronization but does not perform any actions necessary to protect shared resources from concurrent modifications.

Attack Flow

1. An attacker identifies an empty synchronized block.
2. The attacker exploits the race condition by modifying shared resources during the wait period.
3. This leads to data integrity issues and potential system instability.

Prerequisites to Exploit

• Presence of an empty synchronized block in the codebase.
• Multi-threaded execution environment where other threads can modify shared resources during synchronization waits.

Vulnerable Code

synchronized(lock) {}

This code demonstrates a vulnerable pattern where no actions are taken within the synchronized block, leaving shared resources unprotected from concurrent modifications.

Secure Code

synchronized(lock) {
  // Perform necessary operations to protect shared resources.
}

The secure version ensures that any critical operations are performed inside the synchronized block to maintain thread safety and prevent race conditions.

Business Impact of Empty Synchronized Block

Integrity

• Data corruption due to concurrent modifications during synchronization waits.
• Inconsistent application state leading to unpredictable behavior.

Business Consequences:

• Financial losses from system downtime or data corruption.
• Compliance violations if sensitive data is compromised.
• Damage to reputation and loss of customer trust due to unreliable service.

Empty Synchronized Block Attack Scenario

1. An attacker identifies an empty synchronized block in the codebase.
2. The attacker triggers multi-threaded execution, causing other threads to modify shared resources during synchronization waits.
3. This leads to data corruption or inconsistent application state.
4. System instability and potential crashes occur due to race conditions.

How to Detect Empty Synchronized Block

Manual Testing

• Review the codebase for empty synchronized blocks.
• Ensure that all synchronized blocks contain necessary logic.
• Test multi-threaded execution scenarios to identify potential race conditions.

Automated Scanners (SAST / DAST)

Static analysis can detect empty or unnecessary synchronized blocks, while dynamic testing is needed to confirm actual exploitation in runtime environments.

PenScan Detection

PenScan’s ZAP and Wapiti engines are effective at identifying CWE-585 issues during automated scans.

False Positive Guidance

A false positive may occur if the synchronized block contains comments or placeholders that do not represent actual code execution. Ensure that any detected empty blocks are indeed unused before marking them as vulnerabilities.

How to Fix Empty Synchronized Block

• Remove unnecessary synchronized blocks.
• Refactor empty synchronized blocks to include necessary logic for thread safety.
• Implement proper synchronization mechanisms to protect shared resources from concurrent modifications.

Framework-Specific Fixes for Empty Synchronized Block

Java

synchronized(lock) {
  // Perform necessary operations to protect shared resources.
}

Ensure that any critical code is placed within the synchronized block to maintain thread safety and prevent race conditions.

How to Ask AI to Check Your Code for Empty Synchronized Block

Empty Synchronized Block Best Practices Checklist

✅ Ensure all synchronized blocks contain necessary operations to protect shared resources. ✅ Remove or refactor empty synchronized blocks that do not serve a purpose. ✅ Test multi-threaded execution scenarios for potential race conditions. ✅ Use static analysis tools to detect and remove unnecessary synchronized blocks. ✅ Implement proper synchronization mechanisms based on the specific requirements of your application.

Empty Synchronized Block FAQ

How does an empty synchronized block work in Java?

An empty synchronized block waits until no other thread is using the synchronizer, but offers no protection for subsequent code execution.

What are the potential impacts of CWE-585 on a system?

It can lead to race conditions where another thread modifies shared resources while waiting for synchronization, affecting integrity and availability.

How do you detect empty synchronized blocks in your Java codebase?

Use static analysis tools or manual review to identify empty synchronized statements that may not serve their intended purpose.

What is the best practice to avoid CWE-585 in new development?

Ensure all synchronized blocks contain necessary logic and are essential for thread safety when accessing shared resources.

Can you provide an example of vulnerable Java code with empty synchronized block?

An empty synchronized block without subsequent critical code is a potential vulnerability, e.g., synchronized(lock) {}.

How do automated scanners help in identifying CWE-585 issues?

static analysis tools can identify empty or unnecessary synchronized blocks that may not be detected by manual review alone.

What steps should developers take to mitigate CWE-585 after detection?

Remove or refactor the empty synchronized block, ensuring any critical code is properly synchronized and protected.

Vulnerabilities Related to Empty Synchronized Block

References

https://cwe.mitre.org/data/definitions/585.html

• NVD

Scan Your Own Site

Manual code review catches what you know to look for. An automated scan catches what you didn’t. Scan your own website using PenScan to find Empty Synchronized Block and other risks before an attacker does.