CVE-2026-44966: Velocityjs Velocity.js Has A Prototype Pollution Vulnerability

HIGH
Advisory Identifier: GHSA-j658-c2gf-x6pq
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CVSS 8.3
CWE-1321
Published
Advisory Summary

CVE-2026-44966 is a HIGH (CVSS 8.3) velocity.js has a prototype pollution vulnerability (CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')) affecting Velocityjs libraries in the npm ecosystem. The issue involves velocity.js has a prototype pollution vulnerability through #set path assignment. Remediation is available in the latest vendor maintenance releases.

CVE-2026-44966 at a Glance

CVE Identifier CVE-2026-44966
GHSA Identifier GHSA-j658-c2gf-x6pq
Severity HIGH
CVSS v3.1 Base Score 8.3
CWE Classification CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution')
Affected Product Velocityjs
Package Ecosystem npm
Published Date
Last Updated

Security Impact

CVSS: 8.3

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Changed (S:C)
Confidentiality Low (C:L)
Integrity Low (I:L)
Availability Low (A:L)

How to Fix CVE-2026-44966

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

velocityjs fixed: Latest secure release 
npm install velocityjs@Latest secure release
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 velocityjs

How to Check if You Are Affected

To determine whether your Node.js application is affected by CVE-2026-44966, inspect your project dependencies using npm:

npm ls velocityjs

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

Note: The npm ls velocityjs 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 Velocityjs, 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 was discovered in Velocity.js <= 2.1.5. This issue occurs during the processing of #set directives in Velocity templates. If an application renders a template controlled by an attacker, it is possible to modify Object.prototype, potentially leading to Denial of Service (DoS) or Remote Code Execution (RCE) depending on the server environment. ### Details The root cause is located in the #set path assignment logic within the source code: - File: /src/compile/set.ts - Issue: The engine accepts arbitrary path keys and performs assignments using the logic `(baseRef as Record<string, unknown>)[key] = val`. Because there is no validation or filtering to block sensitive keys such as \_\_proto\_\_, constructor, or prototype, an attacker can traverse the prototype chain and pollute the global Object.prototype. ### PoC ```javascript const {render} = require('velocityjs'); delete Object.prototype.polluted; console.log({}.polluted); // "" render('#set($__proto__.polluted = "hacked")', {}); console.log({}.polluted); // "hacked" delete Object.prototype.polluted; ``` ### Impact - Vulnerability Type: Prototype Pollution - Who is impacted: Any application that renders Velocity templates where the template content can be influenced or controlled by untrusted users. - Severity: High. Prototype pollution can often be used to bypass security controls, cause application crashes (DoS), or be chained with other vulnerabilities to achieve code execution.

CVE-2026-44966 Affected Velocityjs Versions

1 component(s)
Product / Package Ecosystem Affected Versions Fixed Version
velocityjs
npm/velocityjs
npm
≤ 2.1.5

References

GitHub Advisory / Wiki ADVISORY
https://github.com/shepherdwind/velocity.js/security/advisories/GHSA-j658-c2gf-x6pq
NVD — CVE-2026-44966 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-44966
Velocityjs Source Repository REPOSITORY
https://github.com/shepherdwind/velocity.js

Frequently Asked Questions

What is CVE-2026-44966?
CVE-2026-44966 is a security vulnerability in Velocityjs involving Velocity.js has a Prototype Pollution vulnerability through #set path assignment under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes ('Prototype Pollution').
Which Velocityjs versions are affected?
Affected versions include releases prior to current release patches across affected packages: velocityjs.
What version fixes CVE-2026-44966?
Upgrading to current release patches (or the latest patched release for your respective branch) resolves the vulnerability.
Is velocityjs affected?
Yes, velocityjs is affected in releases prior to Latest secure release. Upgrade your dependency to Latest secure release or later.
What is the CVSS score of CVE-2026-44966?
CVE-2026-44966 has a CVSS v3.1 base score of 8.3 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L.
What CWE is associated with CVE-2026-44966?
CVE-2026-44966 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-2026-44966?
Run the dependency tree command: 'npm ls velocityjs'. 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-j658-c2gf-x6pq",
  "modified" : "2026-06-08T23:35:04Z",
  "published" : "2026-05-09T00:40:16Z",
  "aliases" : [ "CVE-2026-44966" ],
  "summary" : "Velocity.js has a Prototype Pollution vulnerability through #set path assignment",
  "details" : "### Summary\nA prototype pollution vulnerability was discovered in Velocity.js <= 2.1.5. This issue occurs during the processing of #set directives in Velocity templates. If an application renders a template controlled by an attacker, it is possible to modify Object.prototype, potentially leading to Denial of Service (DoS) or Remote Code Execution (RCE) depending on the server environment.\n\n### Details\nThe root cause is located in the #set path assignment logic within the source code:\n- File: /src/compile/set.ts \n- Issue: The engine accepts arbitrary path keys and performs assignments using the logic `(baseRef as Record<string, unknown>)[key] = val`.\n\n\nBecause there is no validation or filtering to block sensitive keys such as \\_\\_proto\\_\\_, constructor, or prototype, an attacker can traverse the prototype chain and pollute the global Object.prototype.\n\n### PoC\n```javascript\nconst {render} = require('velocityjs');\ndelete Object.prototype.polluted;\nconsole.log({}.polluted); // \"\"\nrender('#set($__proto__.polluted = \"hacked\")', {});\nconsole.log({}.polluted); // \"hacked\"\ndelete Object.prototype.polluted;\n```\n\n### Impact\n- Vulnerability Type: Prototype Pollution\n- Who is impacted: Any application that renders Velocity templates where the template content can be influenced or controlled by untrusted users.\n- Severity: High. Prototype pollution can often be used to bypass security controls, cause application crashes (DoS), or be chained with other vulnerabilities to achieve code execution.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "velocityjs"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "last_affected" : "2.1.5"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/shepherdwind/velocity.js/security/advisories/GHSA-j658-c2gf-x6pq"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-44966"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/shepherdwind/velocity.js"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1321" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-09T00:40:16Z",
    "nvd_published_at" : "2026-05-26T22:16:43Z"
  }
}