CVE-2026-25521: locutus is vulnerable to 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.

• No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
• EPSS 0.5%, percentile 42: FIRST’s model estimates a 0.5% chance of exploitation activity in the next 30 days, higher than 42% 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 locutus (npm) >= 2.0.12, < 2.0.39, fixed in 2.0.39. Check your lockfile, including transitive dependencies.

What it is

Locutus brings stdlibs of other programming languages to JavaScript for educational purposes. In versions from 2.0.12 to before 2.0.39, a prototype pollution vulnerability exists in locutus. 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 String.prototype. This issue has been patched in version 2.0.39.

From the GitHub advisory

Summary A Prototype Pollution vulnerability exists in the the npm package locutus (>2.0.12). 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 String.prototype. This issue was fixed in version 2.0.39.

Details The vulnerability resides in line 77 to 79 of https://github.com/locutusjs/locutus/blob/main/src/php/strings/parsestr.js where includes() function is used to check whether user provided input contain forbidden strings.

PoC

Steps to reproduce 1. Install latest version of locutus 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:

Weakness type: CWE-1321, CWE-915.

Product: locutus (npm) · Assigned by: GitHub_M · Published: 2026-02-04

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 none, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.50% probability of exploitation activity in the next 30 days; percentile 42.

Timeline

• 2026-02-02 — GitHub-reviewed advisory GHSA-rxrv-835q-v5mh published (critical)
• 2026-02-04 — CVE record published by GitHub_M
• 2026-07-15 — 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-rxrv-835q-v5mh github.com
2. CVE.org record CVE-2026-25521 cve.org
3. NVD entry CVE-2026-25521 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. access.redhat.com access.redhat.com
4. RHBZ#2436950 bugzilla.redhat.com
5. security.access.redhat.com security.access.redhat.com

Critical npm advisories · Today’s Security Digest