secure-json-parse

JSON.parse() drop-in replacement with prototype poisoning protection.

Introduction

Consider this:

> const a = '{"__proto__":{ "b":5}}';
'{"__proto__":{ "b":5}}'

> const b = JSON.parse(a);
{ __proto__: { b: 5 } }

> b.b;
undefined

> const c = Object.assign({}, b);
{}

> c.b
5

The problem is that JSON.parse() retains the __proto__ property as a plain object key. By itself, this is not a security issue. However, as soon as that object is assigned to another or iterated on and values copied, the __proto__ property leaks and becomes the object's prototype.

Install

npm install secure-json-parse

Usage

Pass the option object as a second (or third) parameter for configuring the action to take in case of a bad JSON, if nothing is configured, the default is to throw a SyntaxError.
You can choose which action to perform in case __proto__ is present, and in case constructor is present.

const sjson = require('secure-json-parse')

const goodJson = '{ "a": 5, "b": 6 }'
const badJson = '{ "a": 5, "b": 6, "__proto__": { "x": 7 }, "constructor": {"prototype": {"bar": "baz"} } }'

console.log(JSON.parse(goodJson), sjson.parse(goodJson, { protoAction: 'remove', constructorAction: 'remove' }))
console.log(JSON.parse(badJson), sjson.parse(badJson, { protoAction: 'remove', constructorAction: 'remove' }))

API

sjson.parse(text, [reviver], [options])

Parses a given JSON-formatted text into an object where:

• text - the JSON text string.
• reviver - the JSON.parse() optional reviver argument.
• options - optional configuration object where:
  • protoAction - optional string with one of:
    • 'error' - throw a SyntaxError when a __proto__ key is found. This is the default value.
    • 'remove' - deletes any __proto__ keys from the result object.
    • 'ignore' - skips all validation (same as calling JSON.parse() directly).
  • constructorAction - optional string with one of:
    • 'error' - throw a SyntaxError when a constructor key is found. This is the default value.
    • 'remove' - deletes any constructor keys from the result object.
    • 'ignore' - skips all validation (same as calling JSON.parse() directly).

sjson.scan(obj, [options])

Scans a given object for prototype properties where:

• obj - the object being scanned.
• options - optional configuration object where:
  • protoAction - optional string with one of:
    • 'error' - throw a SyntaxError when a __proto__ key is found. This is the default value.
    • 'remove' - deletes any __proto__ keys from the input obj.
  • constructorAction - optional string with one of:
    • 'error' - throw a SyntaxError when a constructor key is found. This is the default value.
    • 'remove' - deletes any constructor keys from the input obj.

Acknowledgements

This project has been forked from hapijs/bourne. All the credits before the commit 4690682 goes to the hapijs/bourne project contributors. After, the project will be maintained by the Fastify team.

License

Licensed under BSD-3-Clause.