CVE-2026-33864: Convict Convict Has Prototype Pollution Vulnerability

CRITICAL
Advisory Identifier: GHSA-44fc-8fm5-q62h
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CWE-1321
Published
Advisory Summary

CVE-2026-33864 is a CRITICAL convict has prototype pollution vulnerability (CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')) affecting Convict libraries in the npm ecosystem. The issue involves convict has prototype pollution via startswith() function. Remediation is available by upgrading affected packages to version 6.2.5.

CVE-2026-33864 at a Glance

CVE Identifier CVE-2026-33864
GHSA Identifier GHSA-44fc-8fm5-q62h
Severity CRITICAL
CVSS v3.1 Base Score N/A
CWE Classification CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
Affected Product Convict
Package Ecosystem npm
Published Date
Last Updated

How to Fix CVE-2026-33864

To remediate this vulnerability, upgrade all affected Convict dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

convict fixed: 6.2.5 
npm install convict@6.2.5
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

npm ls convict

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-33864, inspect your project dependencies using npm:

npm ls convict

Analyze the terminal output to locate any version of Convict in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 6.2.5), your application is affected.

Note: The npm ls convict command inspects dependencies declared in your package.json and lockfile; it does not perform automated runtime dynamic scanning.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution'), where the product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype. In Convict, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### Summary A prototype pollution vulnerability exists in the latest version of the convict npm package (6.2.4). Despite a previous fix that attempted to mitigate prototype pollution by checking whether user input started with a forbidden key, it is still possible to pollute `Object.prototype` via a crafted input using `String.prototype`. ### Details The vulnerability resides in line 564 of https://github.com/mozilla/node-convict/blob/master/packages/convict/src/main.js where `startsWith()` function is used to check whether user provided input contain forbidden strings. ### PoC #### Steps to reproduce 1. Install latest version of convict using `npm install` or cloning from git 2. Run the following code snippet: ```javascript String.prototype.startsWith = () => false; const convict = require('convict'); let obj = {}; const config = convict(obj); console.log({}.polluted); config.set('constructor.prototype.polluted', 'yes'); console.log({}.polluted); // prints yes -> the patch is 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 convict is used by downstream applications. Any application that processes attacker-controlled input using `convict.set` may be affected. It could potentially lead to the following problems: 1. Authentication bypass 2. Denial of service 3. Remote code execution (if polluted property is passed to sinks like eval or child_process)

CVE-2026-33864 Affected Convict Versions

1 component(s)
Fixed Versions:
6.2.5
Product / Package Ecosystem Affected Versions Fixed Version
convict
npm/convict
npm
< 6.2.5
6.2.5

References

GitHub Advisory / Wiki ADVISORY
https://github.com/mozilla/node-convict/security/advisories/GHSA-44fc-8fm5-q62h
Convict Source Repository REPOSITORY
https://github.com/mozilla/node-convict
Convict Source Repository REPOSITORY
https://github.com/mozilla/node-convict/blob/master/packages/convict/src/main.js

Frequently Asked Questions

What is CVE-2026-33864?
CVE-2026-33864 is a security vulnerability in Convict involving Convict has Prototype Pollution via startsWith() function under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution').
Which Convict versions are affected?
Affected versions include releases prior to 6.2.5 across affected packages: convict.
What version fixes CVE-2026-33864?
Upgrading to 6.2.5 (or the latest patched release for your respective branch) resolves the vulnerability.
Is convict affected?
Yes, convict is affected in releases prior to 6.2.5. Upgrade your dependency to 6.2.5 or later.
What CWE is associated with CVE-2026-33864?
CVE-2026-33864 is classified under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution') (The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype.).
How can I check npm dependencies for CVE-2026-33864?
Run the dependency tree command: 'npm ls convict'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-44fc-8fm5-q62h",
  "modified" : "2026-03-26T18:55:41Z",
  "published" : "2026-03-26T18:55:41Z",
  "aliases" : [ "CVE-2026-33864" ],
  "summary" : "Convict has Prototype Pollution via startsWith() function",
  "details" : "### Summary\nA prototype pollution vulnerability exists in the latest version of the convict npm package (6.2.4). Despite a previous fix that attempted to mitigate prototype pollution by checking whether user input started with a forbidden key, it is still possible to pollute `Object.prototype` via a crafted input using `String.prototype`. \n\n### Details\nThe vulnerability resides in line 564 of https://github.com/mozilla/node-convict/blob/master/packages/convict/src/main.js where `startsWith()` function is used to check whether user provided input contain forbidden strings. \n\n### PoC\n#### Steps to reproduce\n1. Install latest version of convict using `npm install` or cloning from git\n2. Run the following code snippet:\n\n```javascript\nString.prototype.startsWith = () => false; \nconst convict = require('convict');\nlet obj = {};\nconst config = convict(obj);\nconsole.log({}.polluted);\nconfig.set('constructor.prototype.polluted', 'yes');\nconsole.log({}.polluted);    // prints yes -> the patch is bypassed and prototype pollution occurred\n```\n\n#### Expected behavior\nPrototype pollution should be prevented and {} should not gain new properties.\nThis should be printed on the console:\n```\nundefined\nundefined OR throw an Error\n```\n\n#### Actual behavior\n`Object.prototype` is polluted \nThis is printed on the console:\n```\nundefined \nyes\n```\n\n### Impact\nThis is a prototype pollution vulnerability, which can have severe security implications depending on how convict is used by downstream applications. Any application that processes attacker-controlled input using `convict.set`  may be affected.\nIt could potentially lead to the following problems:\n\n1. Authentication bypass\n2. Denial of service\n3. Remote code execution (if polluted property is passed to sinks like eval or child_process)",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "convict"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "6.2.5"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 6.2.4"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/mozilla/node-convict/security/advisories/GHSA-44fc-8fm5-q62h"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/mozilla/node-convict"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/mozilla/node-convict/blob/master/packages/convict/src/main.js"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1321" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-26T18:55:41Z",
    "nvd_published_at" : null
  }
}