Velocity.js has a Prototype Pollution vulnerability through #set path assignment

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

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.