GHSA-9c47-m6qq-7p4h Prototype Pollution in JSON5 via Parse Method

• npm json5: >= 2.0.0, < 2.2.2 → fixed in 2.2.2 • npm json5: < 1.0.2 → fixed in 1.0.2

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...'); } ```