CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

Definição

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.

Impacto no mundo real

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.

Como os atacantes a exploram

Trajeto do atacante passo a passo

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.

Exemplo de código vulnerável

Vulnerable JavaScript

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

Vulnerável 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;
  	 }

Exemplo de código seguro

Secure JavaScript

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

Seguro 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.

Lista de verificação de prevenção

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.

Sinais de deteção

How to detect CWE-1321

SAST High

Executar análise estática (SAST) na base de código à procura do padrão inseguro no fluxo de dados.

DAST Moderate

Executar testes dinâmicos de segurança de aplicações (DAST) contra o endpoint em execução.

Runtime Moderate

Monitorizar os registos em tempo de execução para traços de exceção invulgares, input malformado ou tentativas de contornar a autorização.

Code review Moderate

Revisão de código: sinalizar qualquer novo código que trate input desta superfície sem usar os ajudantes validados do framework.

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.

Request AI Swarm Pentest

Perguntas frequentes

Frequently asked questions

O que é o 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.

Qual a gravidade do CWE-1321?

A MITRE não publicou uma classificação de probabilidade de exploração para esta fraqueza. Trate-a como impacto médio até o seu modelo de ameaças provar o contrário.

Que linguagens ou plataformas são afetadas pelo CWE-1321?

MITRE lists the following affected platforms: JavaScript.

Como posso prevenir o 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.

Como é que o Plexicus deteta e corrige o CWE-1321?

O motor SAST do Plexicus correlaciona a assinatura de fluxo de dados do CWE-1321 em cada commit. Quando é encontrada uma correspondência, o nosso agente Codex Remedium abre um PR de correção com o código corrigido, testes e um resumo de uma linha para o revisor.

Onde posso saber mais sobre o CWE-1321?

A MITRE publica a definição canónica em https://cwe.mitre.org/data/definitions/1321.html. Pode também consultar a documentação da OWASP e do NIST para orientações adjacentes.

Fraquezas relacionadas

Weaknesses related to CWE-1321

CWE-915 Pai

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 Pode preceder

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.

Recursos

Pronto para validar o que importa.

O Plexicus é Proof-Driven AppSec: achados validados, compreensão contextual e remediação revisada — ancorada em evidência, escopada com você.

Reservar Pentest

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