CodeQL documentation

Prototype-polluting assignment

ID: js/prototype-polluting-assignment
Kind: path-problem
Severity: warning
Precision: high
   - security
   - external/cwe/cwe-078
   - external/cwe/cwe-079
   - external/cwe/cwe-094
   - external/cwe/cwe-400
   - external/cwe/cwe-915
Query suites:
   - javascript-code-scanning.qls
   - javascript-security-extended.qls
   - javascript-security-and-quality.qls

Click to see the query in the CodeQL repository

Most JavaScript objects inherit the properties of the built-in Object.prototype object. Prototype pollution is a type of vulnerability in which an attacker is able to modify Object.prototype. Since most objects inherit from the compromised Object.prototype object, the attacker can use this to tamper with the application logic, and often escalate to remote code execution or cross-site scripting.

One way to cause prototype pollution is by modifying an object obtained via a user-controlled property name. Most objects have a special __proto__ property that refers to Object.prototype. An attacker can abuse this special property to trick the application into performing unintended modifications of Object.prototype.


Use an associative data structure that is resilient to untrusted key values, such as a Map. In some cases, a prototype-less object created with Object.create(null) may be preferable.

Alternatively, restrict the computed property name so it can’t clash with a built-in property, either by prefixing it with a constant string, or by rejecting inputs that don’t conform to the expected format.


In the example below, the untrusted value is used as the property name req.session.todos[id]. If a malicious user passes in the ID value __proto__, the variable todo will then refer to Object.prototype. Finally, the modification of todo then allows the attacker to inject arbitrary properties onto Object.prototype.

let express = require('express');

express.put('/todos/:id', (req, res) => {
    let id =;
    let items = req.session.todos[id];
    if (!items) {
        items = req.session.todos[id] = {};
    items[] = req.query.text;

One way to fix this is to use Map objects to associate key/value pairs instead of regular objects, as shown below:

let express = require('express');

express.put('/todos/:id', (req, res) => {
    let id =;
    let items = req.session.todos.get(id);
    if (!items) {
        items = new Map();
        req.sessions.todos.set(id, items);
    items.set(, req.query.text);