CWE-1321 Variant Incomplete

Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

Prototype pollution occurs when an application takes user-supplied input and uses it to improperly modify the properties of a JavaScript object's prototype. This allows attackers to inject key-value…

Definition

What is CWE-1321?

Prototype pollution occurs when an application takes user-supplied input and uses it to improperly modify the properties of a JavaScript object's prototype. This allows attackers to inject key-value pairs into the base object, potentially altering the application's logic, crashing it, or escalating privileges.
This vulnerability typically arises in JavaScript when functions like `Object.assign()`, `merge()`, or `deepClone()` recursively combine objects without properly validating the source keys. Attackers can send crafted input containing special keys like `__proto__` or `constructor.prototype`. If this input is merged unsafely, the polluted properties are added to the global prototype chain, affecting every object that inherits from it. To prevent prototype pollution, developers should adopt security-focused practices. These include using objects without prototypes (e.g., `Object.create(null)`), employing safe merge functions that reject prototype-related keys, and validating all external input before processing. Freezing the `Object.prototype` in critical environments can also serve as a defensive measure against this type of attack.
Vulnerability Diagram CWE-1321
Prototype Pollution Attacker JSON { "__proto__": { "isAdmin": true } } Object.prototype deepMerge(obj, input) ↓ writes to __proto__ isAdmin = true on the prototype → inherited by every {} Every object o.isAdmin === true auth bypass / RCE Writing to __proto__ taints every object that inherits from Object.prototype.
Real-world impact

Real-world CVEs caused by CWE-1321

  • CVE-2018-3721

    Prototype pollution by merging objects.

  • CVE-2019-10744

    Prototype pollution by setting default values to object attributes recursively.

  • CVE-2019-11358

    Prototype pollution by merging objects recursively.

  • CVE-2020-8203

    Prototype pollution by setting object attributes based on dot-separated path.

How attackers exploit it

Step-by-step attacker path

  1. 1

    This function sets object attributes based on a dot-separated path.

  2. 2

    This function does not check if the attribute resolves to the object prototype. These codes can be used to add "isAdmin: true" to the object prototype.

  3. 3

    By using a denylist of dangerous attributes, this weakness can be eliminated.

Vulnerable code example

Vulnerable JavaScript

This function sets object attributes based on a dot-separated path.

Vulnerable JavaScript 
function setValueByPath (object, path, value) {
  	 const pathArray = path.split(".");
  	 const attributeToSet = pathArray.pop();
  	 let objectToModify = object;
  	 for (const attr of pathArray) {
  		if (typeof objectToModify[attr] !== 'object') {
  			objectToModify[attr] = {};
  			 }
  		 objectToModify = objectToModify[attr];
  		 }
  	 objectToModify[attributeToSet] = value;
  	 return object;
  	 }
Secure code example

Secure JavaScript

By using a denylist of dangerous attributes, this weakness can be eliminated.

Secure JavaScript 
function setValueByPath (object, path, value) {
  	 const pathArray = path.split(".");
  	 const attributeToSet = pathArray.pop();
  	 let objectToModify = object;
  	 for (const attr of pathArray) {
```
// Ignore attributes which resolve to object prototype* 
  		 if (attr === "__proto__" || attr === "constructor" || attr === "prototype") {
  		
  		```
  			 continue;
  			 }
  		 if (typeof objectToModify[attr] !== "object") {
  			 objectToModify[attr] = {};
  			 }
  		 objectToModify = objectToModify[attr];
  		 }
  	 objectToModify[attributeToSet] = value;
  	 return object;
  	 }
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-1321

  • Implementation By freezing the object prototype first (for example, Object.freeze(Object.prototype)), modification of the prototype becomes impossible.
  • Architecture and Design By blocking modifications of attributes that resolve to object prototype, such as proto or prototype, this weakness can be mitigated.
  • Implementation When handling untrusted objects, validating using a schema can be used.
  • Implementation By using an object without prototypes (via Object.create(null) ), adding object prototype attributes by accessing the prototype via the special attributes becomes impossible, mitigating this weakness.
  • Implementation Map can be used instead of objects in most cases. If Map methods are used instead of object attributes, it is not possible to access the object prototype or modify it.
Detection signals

How to detect CWE-1321

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-1321 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-1321?

Prototype pollution occurs when an application takes user-supplied input and uses it to improperly modify the properties of a JavaScript object's prototype. This allows attackers to inject key-value pairs into the base object, potentially altering the application's logic, crashing it, or escalating privileges.

How serious is CWE-1321?

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-1321?

MITRE lists the following affected platforms: JavaScript.

How can I prevent CWE-1321?

By freezing the object prototype first (for example, Object.freeze(Object.prototype)), modification of the prototype becomes impossible. By blocking modifications of attributes that resolve to object prototype, such as proto or prototype, this weakness can be mitigated.

How does Plexicus detect and fix CWE-1321?

Plexicus's SAST engine matches the data-flow signature for CWE-1321 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-1321?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/1321.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-1321

CWE-915 Parent

Improperly Controlled Modification of Dynamically-Determined Object Attributes

This vulnerability occurs when an application accepts user input that specifies which object attributes or fields to create or update, but…

CWE-471 Can precede

Modification of Assumed-Immutable Data (MAID)

This vulnerability occurs when an application fails to protect data it assumes cannot be changed, allowing an attacker to alter it.

Resources

Further reading

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