Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
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.
Real-world impact
Real-world CVEs caused by CWE-1321
-
Prototype pollution by merging objects.
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Prototype pollution by setting default values to object attributes recursively.
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Prototype pollution by merging objects recursively.
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Prototype pollution by setting object attributes based on dot-separated path.
How attackers exploit it
Step-by-step attacker path
- 1
This function sets object attributes based on a dot-separated path.
- 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
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.
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.
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
Run dynamic application security testing against the live endpoint.
Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.
Code review: flag any new code that handles input from this surface without using the validated framework helpers.
CWE-1321
Don't catalog this weakness. Prove it's reachable.
Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.
Frequently asked questions What is CWE-1321?
How serious is CWE-1321?
What languages or platforms are affected by CWE-1321?
How can I prevent CWE-1321?
How does Plexicus detect and fix CWE-1321?
Where can I learn more about CWE-1321?
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
- MITRE — official CWE-1321 https://cwe.mitre.org/data/definitions/1321.html
- Prototype pollution attack in NodeJS application https://github.com/HoLyVieR/prototype-pollution-nsec18/blob/master/paper/JavaScript_prototype_pollution_attack_in_NodeJS.pdf
- What is Prototype Pollution? https://codeburst.io/what-is-prototype-pollution-49482fc4b638
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