CWE-820: Missing Synchronization

Definition

What is CWE-820?

This vulnerability occurs when multiple parts of your application (like threads or processes) use the same resource—such as a variable, file, or data structure—without proper coordination to control who accesses it and when.

When different parts of your code run concurrently and touch a shared resource without synchronization, the resource's state can become unpredictable. One thread might read a value while another is halfway through modifying it, or two processes might overwrite each other's changes, leading to corrupted data, crashes, or incorrect calculations that break your application's logic. This lack of coordination creates a race condition window that attackers can potentially exploit. By carefully timing their interactions, an attacker might manipulate the shared resource into an unexpected state that bypasses security checks, leaks sensitive information, or causes the system to behave in unintended and insecure ways.

Real-world impact

Real-world CVEs caused by CWE-820

No public CVE references are linked to this CWE in MITRE's catalog yet.

How attackers exploit it

Step-by-step attacker path

1
The following code intends to fork a process, then have both the parent and child processes print a single line.
2
One might expect the code to print out something like:
3
``` PARENT child ```
4
However, because the parent and child are executing concurrently, and stdout is flushed each time a character is printed, the output might be mixed together, such as:
5
``` PcAhRiElNdT [blank line] [blank line] ```

Vulnerable code example

Vulnerable C

The following code intends to fork a process, then have both the parent and child processes print a single line.

Vulnerable C

static void print (char * string) {
  		char * word;
  		int counter;
  		for (word = string; counter = *word++; ) {
  				putc(counter, stdout);
  				fflush(stdout);
```
/* Make timing window a little larger... */* 
  				
  				sleep(1);}}
  
  int main(void) {
  ```
  		pid_t pid;
  		pid = fork();
  		if (pid == -1) {
  			exit(-2);
  		}
  		else if (pid == 0) {
  			print("child\n");
  		}
  		else {
  			print("PARENT\n");
  		}
  		exit(0);
  }

Secure code example

Secure pseudo

// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
  const safe = validateAndEscape(input);
  return executeWithGuards(safe);
}

What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.

Prevention checklist

How to prevent CWE-820

Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
Implementation Validate input at trust boundaries; use allowlists, not denylists.
Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
Operation Monitor logs for the runtime signals listed in the next section.

Detection signals

How to detect CWE-820

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

CWE-820

Don't catalog this weakness. Prove it's reachable.

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Frequently asked questions

What is CWE-820?

This vulnerability occurs when multiple parts of your application (like threads or processes) use the same resource—such as a variable, file, or data structure—without proper coordination to control who accesses it and when.

How serious is CWE-820?

MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.

What languages or platforms are affected by CWE-820?

MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.

How can I prevent CWE-820?

Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.

How does Plexicus detect and fix CWE-820?

Plexicus's SAST engine matches the data-flow signature for CWE-820 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.

Where can I learn more about CWE-820?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/820.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-820

CWE-662 Parent

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Missing Synchronization

What is CWE-820?

Real-world CVEs caused by CWE-820

Step-by-step attacker path

Vulnerable C

Secure pseudo

How to prevent CWE-820

How to detect CWE-820

Don't catalog this weakness. Prove it's reachable.

Frequently asked questions

Weaknesses related to CWE-820

Improper Synchronization

Invokable Control Element in Multi-Thread Context with non-Final Static Storable or Member Element

Singleton Class Instance Creation without Proper Locking or Synchronization

Race Condition within a Thread

Use of Singleton Pattern Without Synchronization in a Multithreaded Context

Unsynchronized Access to Shared Data in a Multithreaded Context

Use of a Non-reentrant Function in a Concurrent Context

Improper Locking

Multiple Locks of a Critical Resource

Further reading

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Sample evidence handover

Server-Side Request Forgery in `webhooks/receiver`