CVE-2026-44966

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

const {render} = require('velocityjs');
delete Object.prototype.polluted;
console.log({}.polluted); // ""
render('#set($__proto__.polluted = "hacked")', {});
console.log({}.polluted); // "hacked"
delete Object.prototype.polluted;

• 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 has a CVSS score of 8.3 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.

Kodem intelligence

1. What is CVE-2026-44966? CVE-2026-44966 is a high-severity security vulnerability in velocityjs (npm), affecting versions <= 2.1.5. No fixed version is listed yet.
2. How severe is CVE-2026-44966? CVE-2026-44966 has a CVSS score of 8.3 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of velocityjs are affected by CVE-2026-44966? velocityjs (npm) versions <= 2.1.5 is affected.
4. Is there a fix for CVE-2026-44966? No fixed version is listed for CVE-2026-44966 yet. Monitor the advisory for updates and apply mitigations in the interim.
5. Is CVE-2026-44966 exploitable, and should I be worried? Whether CVE-2026-44966 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-44966 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.