CVE-2026-27837

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.

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.

CVE-2026-27837 has a CVSS score of 6.3 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.0.7); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-27837? CVE-2026-27837 is a medium-severity security vulnerability in dottie (npm), affecting versions >= 2.0.4, <= 2.0.6. It is fixed in 2.0.7.
2. How severe is CVE-2026-27837? CVE-2026-27837 has a CVSS score of 6.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of dottie are affected by CVE-2026-27837? dottie (npm) versions >= 2.0.4, <= 2.0.6 is affected.
4. Is there a fix for CVE-2026-27837? Yes. CVE-2026-27837 is fixed in 2.0.7. Upgrade to this version or later.
5. Is CVE-2026-27837 exploitable, and should I be worried? Whether CVE-2026-27837 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-27837 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-27837? Upgrade dottie to 2.0.7 or later.