CWE-820 Base Incomplete

Missing Synchronization

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…

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. 1

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

  2. 2

    One might expect the code to print out something like:

  3. 3

    ``` PARENT child ```

  4. 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. 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

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.

Plexicus auto-fix

Plexicus auto-detects CWE-820 and opens a fix PR in under 60 seconds.

Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.

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Frequently asked questions

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

Improper Synchronization

This vulnerability occurs when a multi-threaded or multi-process application allows shared resources to be accessed by multiple threads or…

CWE-1058 Sibling

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

This happens when a method or function, designed to run in a multi-threaded environment, accesses or modifies a non-final static variable…

CWE-1096 Sibling

Singleton Class Instance Creation without Proper Locking or Synchronization

This flaw occurs when a Singleton class is implemented without proper thread-safe controls, allowing multiple instances to be created in…

CWE-366 Sibling

Race Condition within a Thread

This vulnerability occurs when two or more threads within the same application access and manipulate a shared resource (like a variable,…

CWE-543 Sibling

Use of Singleton Pattern Without Synchronization in a Multithreaded Context

This vulnerability occurs when a singleton pattern is implemented in a multithreaded application without proper synchronization,…

CWE-567 Sibling

Unsynchronized Access to Shared Data in a Multithreaded Context

This vulnerability occurs when multiple threads in an application can read and modify shared data, like static variables, without proper…

CWE-663 Sibling

Use of a Non-reentrant Function in a Concurrent Context

This vulnerability occurs when a program uses a function that is not safe for reentrancy within a concurrent environment, such as…

CWE-667 Sibling

Improper Locking

This vulnerability occurs when a program fails to correctly acquire or release a lock on a shared resource, such as a file, database…

CWE-764 Sibling

Multiple Locks of a Critical Resource

This vulnerability occurs when a critical resource, such as a file, data structure, or connection, is locked more times than the software…

Resources

Further reading

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