Security

What is Improper Synchronization (CWE-662)?

Learn how improper synchronization vulnerabilities work, see real-world code examples, and find framework-specific fixes to prevent CWE-662.

Shreya Pillai July 29, 2026 5 min read Security

AI-friendly summary

What it is: Improper Synchronization (CWE-662) is a vulnerability that occurs when multiple threads access shared resources without proper synchronization, leading to race conditions.

Why it matters: This can cause data corruption and unauthorized modifications, compromising application integrity and confidentiality.

How to fix it: Use industry-standard APIs for thread-safe operations and ensure proper locking mechanisms are in place.

TL;DR: Improper Synchronization (CWE-662) is a vulnerability that occurs when shared resources are accessed by multiple threads without proper synchronization, leading to data corruption. Fix it by using standard synchronization APIs.

Field	Value
CWE ID	CWE-662
OWASP Category	Not directly mapped
CAPEC	CAPEC-25, CAPEC-26, CAPEC-27, CAPEC-29
Typical Severity	High
Affected Technologies	multi-threaded applications, concurrent programming
Detection Difficulty	Moderate
Last Updated	2026-07-29

What is Improper Synchronization?

Improper Synchronization (CWE-662) is a type of vulnerability that occurs when multiple threads or processes access shared resources without proper synchronization. As defined by the MITRE Corporation under CWE-662, and classified by the OWASP Foundation under broader categories like security misconfiguration or broken access control…

Quick Summary

Improper Synchronization can cause data corruption, unauthorized modifications, and inconsistent application state due to race conditions. This vulnerability is critical in multi-threaded applications where shared resources are accessed concurrently without proper coordination.

Improper Synchronization Overview

What: Improper Synchronization is a vulnerability where multiple threads or processes access shared resources without proper synchronization, leading to race conditions and data corruption.

Why it matters: It can cause unauthorized modifications, read of application data, altering execution logic, and integrity issues. These vulnerabilities are critical in multi-threaded applications.

Where it occurs: In any environment that uses multi-threading or concurrency, such as Java EE/EJB, Spring, ASP.NET, Django, etc.

Who is affected: Developers and organizations using multi-threaded programming languages and frameworks where shared resources can be accessed concurrently without proper coordination.

Who is NOT affected: Applications that do not use multi-threading or have properly synchronized access to shared resources.

How Improper Synchronization Works

Root Cause

Improper synchronization occurs when multiple threads or processes attempt to simultaneously access a shared resource without appropriate locking mechanisms, leading to race conditions and data corruption.

Attack Flow

An attacker initiates two concurrent requests that target the same shared resource.
The system fails to properly synchronize these requests, allowing both to access the resource simultaneously.
This simultaneous access leads to inconsistent state or data corruption.

Prerequisites to Exploit

Multiple threads or processes accessing a shared resource concurrently.
Lack of proper synchronization mechanisms (e.g., mutexes, semaphores).

Vulnerable Code

import threading

shared_resource = 0

def update_resource(value):
    global shared_resource
    # No synchronization mechanism here
    shared_resource += value

threads = []
for i in range(100):
    t = threading.Thread(target=update_resource, args=(i,))
    threads.append(t)
    t.start()

for thread in threads:
    thread.join()

Secure Code

import threading

shared_resource = 0
lock = threading.Lock()

def update_resource(value):
    global shared_resource
    with lock:
        shared_resource += value

threads = []
for i in range(100):
    t = threading.Thread(target=update_resource, args=(i,))
    threads.append(t)
    t.start()

for thread in threads:
    thread.join()

Business Impact of Improper Synchronization

Confidentiality: Unauthorized access to sensitive data due to race conditions.

Example: An attacker modifies a shared resource before it is read by another process, leading to unauthorized disclosure.

Integrity: Data corruption and inconsistent state due to simultaneous accesses.

Example: Two threads increment the same counter simultaneously, resulting in an incorrect final value.

Availability: Potential disruption of services if critical resources are corrupted or locked indefinitely.

Example: A race condition causes a service to fail repeatedly, leading to downtime.

Business Consequences

Financial losses due to service disruptions and data corruption.
Compliance violations for failing to protect sensitive information.
Reputational damage from publicized security incidents.

Improper Synchronization Attack Scenario

An attacker initiates multiple concurrent requests targeting a shared resource.
The system fails to properly synchronize these requests, allowing simultaneous access.
This leads to inconsistent state or data corruption in the shared resource.
As a result, the application behaves unpredictably and may expose sensitive information.

How to Detect Improper Synchronization

Manual Testing

Review code for shared resource access without appropriate locks or synchronization primitives.
Check if critical sections of code are properly enclosed within locking mechanisms (mutexes, semaphores).
Verify that thread-safe APIs and libraries are used where necessary.

Automated Scanners (SAST / DAST)

Static analysis can detect potential race conditions by analyzing code for shared resource access without proper synchronization. Dynamic testing requires runtime observation of concurrent processes to identify actual instances of improper synchronization.

PenScan Detection

PenScan uses advanced static analysis tools like ZAP and Nuclei to identify improper synchronization patterns in code.

False Positive Guidance

A real finding would show simultaneous access to a shared resource without proper locking mechanisms, while false positives may occur if the pattern is present but correctly synchronized by context not visible to the scanner.

How to Fix Improper Synchronization

Use industry-standard APIs for thread-safe operations.
Ensure proper locking mechanisms are in place (mutexes, semaphores).
Employ atomic operations where possible to avoid race conditions.
Implement proper synchronization primitives to manage concurrent access to shared resources.

Framework-Specific Fixes for Improper Synchronization

Java

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class Example {
    private final Lock lock = new ReentrantLock();
    private int sharedResource = 0;

    public void updateResource(int value) {
        lock.lock();
        try {
            sharedResource += value;
        } finally {
            lock.unlock();
        }
    }
}

Python

import threading

shared_resource = 0
lock = threading.Lock()

def update_resource(value):
    global shared_resource
    with lock:
        shared_resource += value

threads = []
for i in range(100):
    t = threading.Thread(target=update_resource, args=(i,))
    threads.append(t)
    t.start()

for thread in threads:
    thread.join()

Node.js

const { Mutex } = require('async-mutex');

const mutex = new Mutex();
let sharedResource = 0;

function updateResource(value) {
    const release = await mutex.acquire();
    try {
        sharedResource += value;
    } finally {
        release();
    }
}

PHP

use RedBeanPHP\RedBeanFacade as R;

$mutex = new \Mutex();

function updateResource($value) {
    $mutex->lock('shared_resource');
    global $sharedResource;
    $sharedResource += $value;
    $mutex->unlock();
}

How to Ask AI to Check Your Code for Improper Synchronization

Review the following [language] code block for potential CWE-662 Improper Synchronization vulnerabilities and rewrite it using proper synchronization mechanisms: [paste code here]

Copy-paste prompt

Review the following [language] code block for potential CWE-662 Improper Synchronization vulnerabilities and rewrite it using proper synchronization mechanisms: [paste code here]

Improper Synchronization Best Practices Checklist

✅ Use industry-standard APIs to synchronize access to shared resources. ✅ Implement proper locking mechanisms (mutexes, semaphores) around critical sections of code. ✅ Employ atomic operations where possible to avoid race conditions. ✅ Ensure that thread-safe libraries and frameworks are used in multi-threaded applications. ✅ Regularly review and test synchronization logic during development cycles.

Improper Synchronization FAQ

How does improper synchronization occur in multi-threaded applications?

Improper synchronization occurs when multiple threads access a shared resource without proper coordination, leading to race conditions or data corruption.

What are the common consequences of improper synchronization?

Improper synchronization can lead to unauthorized modification and read of application data, altering execution logic, and causing integrity issues.

How does OWASP categorize improper synchronization?

While there is no direct mapping in the current OWASP Top 10, improper synchronization falls under broader categories like security misconfiguration or broken access control.

What are some common mitigation strategies for improper synchronization?

Use industry-standard APIs to synchronize code and ensure proper locking mechanisms are in place to prevent simultaneous accesses by multiple threads.

How can I detect improper synchronization vulnerabilities manually?

Review code for shared resource access without appropriate locks or synchronization primitives, such as mutexes or semaphores.

What is the impact of a race condition on application integrity?

A race condition can cause data corruption and inconsistent state, leading to unpredictable behavior and potential security vulnerabilities.

How does PenScan detect improper synchronization issues?