Vulnerability-Lookup

GHSA-VC8F-X9PP-WF5P

Vulnerability from github – Published: 2026-03-27 17:58 – Updated: 2026-03-30 20:15

Summary

Locutus Prototype Pollution due to incomplete fix for CVE-2026-25521

Details

Summary

A prototype pollution vulnerability exists in the parse_str function of the npm package locutus. An attacker can pollute Object.prototype by overriding RegExp.prototype.test and then passing a crafted query string to parse_str, bypassing the prototype pollution guard.

This vulnerability stems from an incomplete fix for CVE-2026-25521. The CVE-2026-25521 patch replaced the String.prototype.includes()-based guard with a RegExp.prototype.test()-based guard. However, RegExp.prototype.test is itself a writable prototype method that can be overridden, making the new guard bypassable in the same way as the original — trading one hijackable built-in for another.

Package

locutus (npm)

Affected versions

= 2.0.39, <= 3.0.24

Tested and confirmed vulnerable on 2.0.39 and 3.0.24 (latest). Version 2.0.38 (pre-fix) uses a different guard (String.prototype.includes) and is not affected by this specific bypass.

Description

Details

The vulnerability resides in parse_str.js where the RegExp.prototype.test() function is used to check whether user-provided input contains forbidden keys:

if (/__proto__|constructor|prototype/.test(key)) {
  break
}

The previous guard (fixed in CVE-2026-25521) used String.prototype.includes():

if (key.includes('__proto__')) {
  break
}

The CVE-2026-25521 fix correctly identified that String.prototype.includes can be hijacked. However, the replacement guard using RegExp.prototype.test() suffers from the same class of weakness — RegExp.prototype.test is a writable method on the prototype chain and can be overridden to always return false, completely disabling the guard.

The robust fix is to use direct string comparison operators (===) in native control flow (for/if) instead of prototype methods like RegExp.prototype.test(), since === is a language-level operator that cannot be overridden.

PoC

Steps to reproduce

Install locutus using npm install locutus
Run the following code snippet:

const parse_str = require('locutus/php/strings/parse_str');

// Hijack RegExp.prototype.test (simulates a prior prototype pollution gadget)
const original = RegExp.prototype.test;
RegExp.prototype.test = function () { return false; };

// Payload
const result = {};
parse_str('__proto__[polluted]=yes', result);

// Check
RegExp.prototype.test = original;
console.log(({}).polluted); // 'yes' — prototype is polluted

Expected behavior

Prototype pollution should be prevented and ({}).polluted should print undefined.

undefined

Actual behavior

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

yes

Impact

This is a prototype pollution vulnerability with the same impact as CVE-2026-25521. The attack requires a chaining scenario — an attacker needs a separate prototype pollution gadget (e.g., from another npm package in the same application) to override RegExp.prototype.test before exploiting parse_str. This is realistic in Node.js applications that use multiple npm packages, where one package's vulnerability can disable another package's defenses.

Any application that processes attacker-controlled input using locutus/php/strings/parse_str may be affected. It could potentially lead to:

Authentication bypass
Denial of service
Remote code execution (if polluted property is passed to sinks like eval or child_process)

Resources

Original advisory: https://github.com/locutusjs/locutus/security/advisories/GHSA-rxrv-835q-v5mh
Fix commit (incomplete): https://github.com/locutusjs/locutus/commit/042af9ca7fde2ff599120783e720a17f335bb01c
Vulnerable file: https://github.com/locutusjs/locutus/blob/main/src/php/strings/parse_str.js#L77

Maintainer response

Thank you for the follow-up report. This issue was reproduced locally against locutus@3.0.24, confirming that the earlier parse_str guard was incomplete: if RegExp.prototype.test was already compromised, the guard could be bypassed and parse_str('__proto__[polluted]=yes', result) could still pollute Object.prototype.

This is now fixed on main and released in locutus@3.0.25.

Fix Shipped In

PR: locutusjs/locutus#597
Merge commit on main: 345a6211e1e6f939f96a7090bfeff642c9fcf9e4
Release: v3.0.25

What the Fix Does

The new fix no longer relies on a regex-prototype guard for safety. Instead, src/php/strings/parse_str.ts now rejects dangerous key paths during parsed-segment assignment, so the sink itself is hardened even if RegExp.prototype.test has been tampered with beforehand.

Tested Repro Before the Fix

Override RegExp.prototype.test to always return false
Call parse_str('__proto__[polluted]=yes', result)
Observe ({}).polluted === 'yes'

Tested State After the Fix in `3.0.25`

Dangerous key paths are skipped during assignment
The same chained repro no longer pollutes Object.prototype
The regression is covered by test/custom/parse_str-prototype-pollution.vitest.ts

The locutus team is treating this as a real package vulnerability with patched version 3.0.25. The vulnerable range should end at < 3.0.25.

Severity

6.3 (Medium)


                  
                    CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "locutus"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.0.39"
            },
            {
              "fixed": "3.0.25"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33994"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1321"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-03-27T17:58:48Z",
    "nvd_published_at": "2026-03-27T23:17:14Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nA prototype pollution vulnerability exists in the `parse_str` function of the npm package locutus. An attacker can pollute `Object.prototype` by overriding `RegExp.prototype.test` and then passing a crafted query string to `parse_str`, bypassing the prototype pollution guard.\n\nThis vulnerability stems from an incomplete fix for [CVE-2026-25521](https://github.com/locutusjs/locutus/security/advisories/GHSA-rxrv-835q-v5mh). The CVE-2026-25521 patch replaced the `String.prototype.includes()`-based guard with a `RegExp.prototype.test()`-based guard. However, `RegExp.prototype.test` is itself a writable prototype method that can be overridden, making the new guard bypassable in the same way as the original \u2014 trading one hijackable built-in for another.\n\n## Package\n\nlocutus (npm)\n\n## Affected versions\n\n\u003e= 2.0.39, \u003c= 3.0.24\n\nTested and confirmed vulnerable on **2.0.39** and **3.0.24** (latest). Version 2.0.38 (pre-fix) uses a different guard (`String.prototype.includes`) and is not affected by this specific bypass.\n\n---\n\n## Description\n\n### Details\n\nThe vulnerability resides in `parse_str.js` where the `RegExp.prototype.test()` function is used to check whether user-provided input contains forbidden keys:\n\n```javascript\nif (/__proto__|constructor|prototype/.test(key)) {\n  break\n}\n```\n\nThe previous guard (fixed in CVE-2026-25521) used `String.prototype.includes()`:\n\n```javascript\nif (key.includes(\u0027__proto__\u0027)) {\n  break\n}\n```\n\nThe CVE-2026-25521 fix correctly identified that `String.prototype.includes` can be hijacked. However, the replacement guard using `RegExp.prototype.test()` suffers from the same class of weakness \u2014 `RegExp.prototype.test` is a writable method on the prototype chain and can be overridden to always return `false`, completely disabling the guard.\n\nThe robust fix is to use direct string comparison operators (`===`) in native control flow (`for`/`if`) instead of prototype methods like `RegExp.prototype.test()`, since `===` is a language-level operator that cannot be overridden.\n\n### PoC\n\n#### Steps to reproduce\n\n1. Install locutus using `npm install locutus`\n2. Run the following code snippet:\n\n```javascript\nconst parse_str = require(\u0027locutus/php/strings/parse_str\u0027);\n\n// Hijack RegExp.prototype.test (simulates a prior prototype pollution gadget)\nconst original = RegExp.prototype.test;\nRegExp.prototype.test = function () { return false; };\n\n// Payload\nconst result = {};\nparse_str(\u0027__proto__[polluted]=yes\u0027, result);\n\n// Check\nRegExp.prototype.test = original;\nconsole.log(({}).polluted); // \u0027yes\u0027 \u2014 prototype is polluted\n```\n\n#### Expected behavior\n\nPrototype pollution should be prevented and `({}).polluted` should print `undefined`.\n\n```\nundefined\n```\n\n#### Actual behavior\n\n`Object.prototype` is polluted. This is printed on the console:\n\n```\nyes\n```\n\n### Impact\n\nThis is a prototype pollution vulnerability with the same impact as CVE-2026-25521. The attack requires a chaining scenario \u2014 an attacker needs a separate prototype pollution gadget (e.g., from another npm package in the same application) to override `RegExp.prototype.test` before exploiting `parse_str`. This is realistic in Node.js applications that use multiple npm packages, where one package\u0027s vulnerability can disable another package\u0027s defenses.\n\nAny application that processes attacker-controlled input using `locutus/php/strings/parse_str` may be affected. It could potentially lead to:\n\n1. Authentication bypass\n2. Denial of service\n3. Remote code execution (if polluted property is passed to sinks like `eval` or `child_process`)\n\n### Resources\n\n- Original advisory: https://github.com/locutusjs/locutus/security/advisories/GHSA-rxrv-835q-v5mh\n- Fix commit (incomplete): https://github.com/locutusjs/locutus/commit/042af9ca7fde2ff599120783e720a17f335bb01c\n- Vulnerable file: https://github.com/locutusjs/locutus/blob/main/src/php/strings/parse_str.js#L77\n\n## Maintainer response\n\nThank you for the follow-up report. This issue was reproduced locally against `locutus@3.0.24`, confirming that the earlier `parse_str` guard was incomplete: if `RegExp.prototype.test` was already compromised, the guard could be bypassed and `parse_str(\u0027__proto__[polluted]=yes\u0027, result)` could still pollute `Object.prototype`.\n\nThis is now fixed on `main` and released in `locutus@3.0.25`.\n\n## Fix Shipped In\n\n- **PR:** [locutusjs/locutus#597](https://github.com/locutusjs/locutus/pull/597)\n- **Merge commit on `main`:** `345a6211e1e6f939f96a7090bfeff642c9fcf9e4`\n- **Release:** [v3.0.25](https://github.com/locutusjs/locutus/releases/tag/v3.0.25)\n\n## What the Fix Does\n\nThe new fix no longer relies on a regex-prototype guard for safety. Instead, `src/php/strings/parse_str.ts` now rejects dangerous key paths during parsed-segment assignment, so the sink itself is hardened even if `RegExp.prototype.test` has been tampered with beforehand.\n\n## Tested Repro Before the Fix\n\n- Override `RegExp.prototype.test` to always return `false`\n- Call `parse_str(\u0027__proto__[polluted]=yes\u0027, result)`\n- Observe `({}).polluted === \u0027yes\u0027`\n\n## Tested State After the Fix in `3.0.25`\n\n- Dangerous key paths are skipped during assignment\n- The same chained repro no longer pollutes `Object.prototype`\n- The regression is covered by `test/custom/parse_str-prototype-pollution.vitest.ts`\n\n---\n\nThe locutus team is treating this as a real package vulnerability with patched version `3.0.25`. The vulnerable range should end at `\u003c 3.0.25`.",
  "id": "GHSA-vc8f-x9pp-wf5p",
  "modified": "2026-03-30T20:15:19Z",
  "published": "2026-03-27T17:58:48Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/locutusjs/locutus/security/advisories/GHSA-rxrv-835q-v5mh"
    },
    {
      "type": "WEB",
      "url": "https://github.com/locutusjs/locutus/security/advisories/GHSA-vc8f-x9pp-wf5p"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-33994"
    },
    {
      "type": "WEB",
      "url": "https://github.com/locutusjs/locutus/pull/597"
    },
    {
      "type": "WEB",
      "url": "https://github.com/locutusjs/locutus/commit/345a6211e1e6f939f96a7090bfeff642c9fcf9e4"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/locutusjs/locutus"
    },
    {
      "type": "WEB",
      "url": "https://github.com/locutusjs/locutus/releases/tag/v3.0.25"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Locutus Prototype Pollution due to incomplete fix for CVE-2026-25521"
}

CVE-2026-33994 (GCVE-0-2026-33994)

Vulnerability from cvelistv5 – Published: 2026-03-27 22:15 – Updated: 2026-04-01 13:45

Title

Locutus Prototype Pollution due to incomplete fix for CVE-2026-25521

Summary

Locutus brings stdlibs of other programming languages to JavaScript for educational purposes. Starting in version 2.0.39 and prior to version 3.0.25, a prototype pollution vulnerability exists in the `parse_str` function of the npm package locutus. An attacker can pollute `Object.prototype` by overriding `RegExp.prototype.test` and then passing a crafted query string to `parse_str`, bypassing the prototype pollution guard. This vulnerability stems from an incomplete fix for CVE-2026-25521. The CVE-2026-25521 patch replaced the `String.prototype.includes()`-based guard with a `RegExp.prototype.test()`-based guard. However, `RegExp.prototype.test` is itself a writable prototype method that can be overridden, making the new guard bypassable in the same way as the original — trading one hijackable built-in for another. Version 3.0.25 contains an updated fix.

Severity

6.3 (Medium)


                        
                          CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N

9.8 (Critical)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

SSVC

Exploitation: poc Automatable: yes Technical Impact: total

CISA Coordinator · CISA-ADP (v2.0.3)

Decision recorded 2026-04-01 13:45 UTC

CWE

CWE-1321 - Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')

Assigner

GitHub_M mitre root Scorecard

References

4 references

URL	Tags
https://github.com/locutusjs/locutus/security/adv…	x_refsource_CONFIRM
https://github.com/locutusjs/locutus/pull/597	x_refsource_MISC
https://github.com/locutusjs/locutus/commit/345a6…	x_refsource_MISC
https://github.com/locutusjs/locutus/releases/tag…	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
locutusjs	locutus	Affected: >= 2.0.39, < 3.0.25

Show details on NVD website

JSON

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  "containers": {
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      {
        "metrics": [
          {
            "cvssV3_1": {
              "attackComplexity": "LOW",
              "attackVector": "NETWORK",
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              "baseScore": 9.8,
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              "confidentialityImpact": "HIGH",
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              "userInteraction": "NONE",
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            }
          },
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                "options": [
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                "role": "CISA Coordinator",
                "timestamp": "2026-04-01T13:45:33.039333Z",
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    ],
    "cna": {
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          "product": "locutus",
          "vendor": "locutusjs",
          "versions": [
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        {
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}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

GHSA-VC8F-X9PP-WF5P

Summary

Package

Affected versions

Description

Details

PoC

Steps to reproduce

Expected behavior

Actual behavior

Impact

Resources

Maintainer response

Fix Shipped In

What the Fix Does

Tested Repro Before the Fix

Tested State After the Fix in 3.0.25

CVE-2026-33994 (GCVE-0-2026-33994)

Tags

Sightings

Nomenclature

Related by attack behaviour

Tested State After the Fix in `3.0.25`