# Cwe 820 Unsynchronized Access

> Use this skill when you need to remediate CWE-820 (Missing Synchronization) vulnerabilities in Java code. Triggers on SAST findings, security reviews, or when fixing missing synchronization issues.

- Skill: `developerscoffee/cwe-820-unsynchronized-access` (Agent Skill)
- Install (CLI): `npx skillmds@latest add developerscoffee/cwe-820-unsynchronized-access`
- Raw SKILL.md: https://api.skillmd.com/api/skills/developerscoffee/cwe-820-unsynchronized-access/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Security
- License: MIT
- Author: DevelopersCoffee (https://skillmd.com/u/developerscoffee)
- Updated: 2026-09-10
- Page: https://skillmd.com/skills/developerscoffee/cwe-820-unsynchronized-access

---


# CWE-820 Missing Synchronization

## Description

Missing Synchronization

Reference:
https://cwe.mitre.org/data/definitions/820.html


**OWASP Category**: A04:2021 – Insecure Design


---

## Vulnerable Pattern


### ❌ Example 1: Vulnerable Pattern

```java
// VULNERABLE: Unsynchronized access to shared state
public class Counter {
    private int count = 0;  // Shared mutable state!

    public void increment() {
        count++;  // Not atomic - race condition!
    }

    public int getCount() {
        return count;
    }
}

// VULNERABLE: Non-thread-safe collection
private Map<String, Session> sessions = new HashMap<>();  // Not thread-safe!

public void addSession(String id, Session session) {
    sessions.put(id, session);  // Concurrent modification risk!
}
```


**Why it's vulnerable:** This pattern is vulnerable to Missing Synchronization




---

## Deterministic Fix


### ✅ Secure Implementation: Secure Implementation

```java
// SECURE: Use AtomicInteger for counters
public class Counter {
    private final AtomicInteger count = new AtomicInteger(0);

    public void increment() {
        count.incrementAndGet();  // Atomic operation
    }

    public int getCount() {
        return count.get();
    }
}

// SECURE: Use ConcurrentHashMap for shared maps
private final Map<String, Session> sessions = new ConcurrentHashMap<>();

public void addSession(String id, Session session) {
    sessions.put(id, session);  // Thread-safe
}

// SECURE: Use synchronized for complex operations
public class Account {
    private BigDecimal balance;
    private final Object lock = new Object();

    public void transfer(Account target, BigDecimal amount) {
        // Lock ordering to prevent deadlock
        Object first = System.identityHashCode(this) < System.identityHashCode(target)
            ? this.lock : target.lock;
        Object second = first == this.lock ? target.lock : this.lock;

        synchronized (first) {
            synchronized (second) {
                this.balance = this.balance.subtract(amount);
                target.balance = target.balance.add(amount);
            }
        }
    }
}
```


**Why it's secure:** Implements proper protection against Missing Synchronization




---

## Detection Pattern

Look for these patterns in your codebase:


```bash
# Find unsynchronized shared fields
grep -rn "private.*Map.*=.*new HashMap\\|private int.*=" --include="*.java"
```



---

## Remediation Steps


1. Use AtomicInteger/AtomicLong for counters

2. Use ConcurrentHashMap instead of HashMap

3. Use Collections.synchronizedMap() if needed

4. Add synchronized blocks for compound operations


---

## Key Imports

```java

import java.util.concurrent.atomic.AtomicInteger;

import java.util.concurrent.ConcurrentHashMap;

```

---

## Verification

After remediation:


- Run SAST scanner to confirm vulnerability is resolved

- Review all instances of the vulnerable pattern

- Add unit tests that verify the secure implementation

- Check for similar patterns in related code


---

## Trigger Examples

```
Fix CWE-820 vulnerability
Resolve Missing Synchronization issue
Secure this Java code against missing synchronization
SAST reports CWE-820
```

---

## Common Vulnerable Locations

| Layer | Files | Patterns |
|-------|-------|----------|

| Controller | `*Controller.java` | User input handling |

| Service | `*Service.java` | Business logic |

| Repository | `*Repository.java` | Data access |


---

## References


- [CWE-820: Missing Synchronization](https://cwe.mitre.org/data/definitions/820.html)


---

**Source**: Generated by [Java CWE Security Skills Generator](https://github.com/DevelopersCoffee/java-cwe-security-skills)
**Last Updated**: 2026-03-07