CVE-2026-26021: set-in Affected by Prototype Pollution

Should you act?

Critical, preconditions apply

Severe if exploited, but exploitation needs conditions that may not apply to you. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.6%, percentile 46: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 46% of all scored CVEs (low). EPSS is a probability, not a verdict.
• Needs local access needed. Lower urgency for most teams unless an attacker could already run code on the machine.
• Only applies if your dependency tree includes set-in (npm) >= 2.0.1, < 2.0.5, fixed in 2.0.5. Check your lockfile, including transitive dependencies.

What it is

set-in provides the set value of nested associative structure given array of keys. A prototype pollution vulnerability exists in the the npm package set-in (>=2.0.1,

From the GitHub advisory

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 1. Install latest version of set-in using npm install or cloning from git 2. Run the following code snippet:

Expected behavior Prototype pollution should be prevented and {} should not gain new properties. This should be printed on the console:

Actual behavior Object.prototype is polluted This is printed on the console:

Weakness type: CWE-1321.

Product: set-in (npm) · Assigned by: GitHub_M · Published: 2026-02-11

Affected and fixed versions

Severity scores

• Attack vector: local access needed
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.56% probability of exploitation activity in the next 30 days; percentile 46.

Timeline

• 2026-02-11 — CVE record published by GitHub_M
• 2026-02-11 — GitHub-reviewed advisory GHSA-2c4m-g7rx-63q7 published (critical)
• 2026-02-12 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-2c4m-g7rx-63q7 github.com
2. CVE.org record CVE-2026-26021 cve.org
3. NVD entry CVE-2026-26021 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com
5. github.com github.com
6. github.com github.com

Critical npm advisories · Today’s Security Digest