defu: `defaults`引数内の`__proto__`キーによるプロトタイプ汚染

Prototype pollution occurs when an attacker can modify the prototype of JavaScript objects, effectively altering the behavior of all objects inheriting from that prototype. In the context of defu, an attacker can inject a proto key into user-supplied data (like JSON request bodies or configuration files) and pass it to the defu() function. This allows them to override intended default values, potentially granting unauthorized access or modifying application logic. For example, an attacker could set isAdmin: true to bypass access controls. The blast radius extends to any part of the application relying on the defu function and its default values, making it a significant security concern.

Applications built with Node.js that utilize the defu package for managing configuration or default values are at risk. This includes web applications, APIs, and backend services. Specifically, applications that parse user-supplied JSON data or load configuration files from untrusted sources are particularly vulnerable.

• nodejs / server:

  npm audit defu

• nodejs / supply-chain:

  find . -name 'package.json' -print0 | xargs -0 grep '"defu": ["^6.1.5"']

• generic web: Inspect application logs for unusual object properties or unexpected behavior after processing user input. Look for instances where default values are being unexpectedly overridden.

1. 2026-04-04Disclosure

   disclosure

1. 予約済み2026-04-01
2. 公開日2026-04-04
3. 更新日2026-04-06
4. EPSS 更新日2026-04-14

The primary mitigation for CVE-2026-35209 is to upgrade to defu version 6.1.5 or later, which includes a fix for the vulnerability. If upgrading is not immediately feasible, implement strict input sanitization on any user-supplied data passed to defu(). This involves validating and filtering input to prevent the injection of malicious proto payloads. Consider using a dedicated input validation library to ensure comprehensive protection. Additionally, review application code that utilizes defu to identify potential areas where prototype pollution could be exploited. After upgrading, confirm the fix by attempting to trigger the prototype pollution vulnerability with a crafted payload and verifying that the default values are not overridden.