CVE-2026-26021 — set-in
Fix: ahdinosaur/set-in@b8e1dabCVE-2026-26021 is a CWE-1321 vulnerability in set-in. A fix is available for set-in — see the affected versions and patch details below.
Prototype pollution in set-in
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-26021.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
set-innpmDescription
Summary
A prototype pollution vulnerability exists in the the npm package set-in (>=2.0.1). Despite a previous fix that attempted to mitigate prototype pollution by checking whether user input contained a forbidden key, it is still possible to pollute Object.prototype via a crafted input using Array.prototype. This has been fixed in version 2.0.5.
Details
The vulnerability resides in line 28 of https://github.com/ahdinosaur/set-in/blob/master/index.js where includes() function is used to check whether user provided input contain forbidden strings.
PoC
Steps to reproduce
- Install latest version of set-in using npm install or cloning from git
- Run the following code snippet:
Array.prototype.includes = () => false;
const si = require('set-in');
const obj = {};
console.log({}.polluted);
si(obj, [
'constructor',
'prototype',
'polluted'
], 'yes');
console.log('{ ' + obj.polluted + ', ' + 'yes' + ' }'); // prints yes -> indicating that the patch was bypassed and prototype pollution occurred
Expected behavior
Prototype pollution should be prevented and {} should not gain new properties. This should be printed on the console:
undefined
undefined OR throw an Error
Actual behavior
Object.prototype is polluted This is printed on the console:
undefined
yes
Impact
This is a prototype pollution vulnerability, which can have severe security implications depending on how set-in is used by downstream applications. Any application that processes attacker-controlled input using this package may be affected. It could potentially lead to the following problems:
- Authentication bypass
- Denial of service
- Remote code execution (if polluted property is passed to sinks like eval or child_process)
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | set-in | ≥ 2.0.1&&< 2.0.5 | 2.0.5npm install set-in@2.0.5 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for set-in, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update set-in to 2.0.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-26021 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-26021 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-26021. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-26021 in your dependencies?
O3 Security finds CVE-2026-26021 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.