Prototype Pollution via parse() in NodeJS flatted

The core of this vulnerability lies in the parse() function's mishandling of attacker-controlled string values. When parsing JSON, the function uses these values as direct array index keys without proper validation. By crafting a malicious JSON payload containing the key proto, an attacker can gain control over the prototype chain of the parsed object. This allows them to inject arbitrary properties and methods into Array.prototype, effectively polluting the global scope. Subsequent code that relies on the modified Array.prototype will inherit these malicious properties, potentially leading to unexpected behavior, denial of service, or even remote code execution. This is similar to other prototype pollution vulnerabilities, but the direct access to Array.prototype amplifies the potential impact.

Applications that utilize the flatted library for JSON parsing, particularly those handling untrusted input, are at risk. Node.js projects, especially those relying on flatted as a dependency, are directly affected. Development environments and continuous integration pipelines using vulnerable versions of flatted are also potential targets.

• nodejs / server:

  # Check for vulnerable flatted versions
  npm list flatted

• nodejs / server:

  # Inspect flatted module in node_modules for suspicious code
  find node_modules/flatted -name '*.js' -print0 | xargs -0 grep -i '__proto__' 

• generic web:

  # Check for unusual properties on Array.prototype in running applications
  # (Requires access to application code or debugging tools)
  console.log(Array.prototype);

1. 2026-03-19Disclosure

   disclosure

1. 予約済み2026-03-18
2. 公開日2026-03-19
3. 更新日2026-03-25
4. EPSS 更新日2026-03-28

The primary mitigation for CVE-2026-33228 is to upgrade to flatted version 3.4.2 or later. This version includes a fix that properly validates array index keys during the parsing process, preventing the prototype pollution attack. If upgrading immediately is not feasible, consider implementing input validation on the JSON data being parsed by flatted. Strictly enforce that keys are numeric or adhere to a predefined whitelist. Additionally, consider using a Web Application Firewall (WAF) to filter out malicious JSON payloads containing proto or other suspicious keys. After upgrading, confirm the fix by attempting to parse a malicious JSON payload containing proto and verifying that it no longer modifies the global prototype.

最終更新

3.4.22ヶ月前