dottie is vulnerable to Prototype Pollution bypass via non-first path segments in set() and transform()

Summary

dottie versions 2.0.4 through 2.0.6 contain an incomplete fix for CVE-2023-26132. The prototype pollution guard introduced in commit 7d3aee1 only validates the first segment of a dot-separated path, allowing an attacker to bypass the protection by placing __proto__ at any position other than the first.

Both dottie.set() and dottie.transform() are affected.

Details

The existing guard checks only pieces[0] === '__proto__'. When a path like 'a.__proto__.polluted' is used, pieces[0] evaluates to 'a', not '__proto__', so the guard is bypassed.

Inside the traversal loop, current['__proto__'] = {} triggers the __proto__ setter, replacing the intermediate object's prototype. The final value is then written onto this new prototype.

Important distinction: This vulnerability does NOT pollute the global Object.prototype. It injects properties into a specific object's prototype chain. However, injected properties are invisible to hasOwnProperty() and Object.keys(), which makes them difficult to detect and can lead to authorization bypass in common coding patterns.

PoC

const dottie = require('dottie');

// set() bypass
const obj = {};
dottie.set(obj, 'session.__proto__.isAdmin', true);
console.log(obj.session.isAdmin);                    // true
console.log(({}).isAdmin);                           // undefined
console.log(obj.session.hasOwnProperty('isAdmin'));  // false

// transform() bypass
const flat = { 'user.__proto__.role': 'admin', 'user.name': 'guest' };
const result = dottie.transform(flat);
console.log(result.user.role);                       // 'admin'
console.log(({}).role);                              // undefined

Tested on Node.js v20 and v22, dottie 2.0.6, Windows 11.

Impact

The primary risk is authorization bypass. In a typical server-side scenario where dottie is used to process user input (e.g., via Sequelize, which depends on dottie with ~1.3M weekly npm downloads), an attacker can inject properties like isAdmin: true into objects used for access control decisions. Since the injected property is not an own property, standard checks using hasOwnProperty() or Object.keys() will not reveal it, while property access like if (session.isAdmin) will return true.

Additionally, replacing an object's prototype via current['__proto__'] = {} strips all inherited methods, potentially causing TypeError exceptions and denial of service.