CVE-2023-3696: Mongoosejs Mongoose Prototype Pollution

CVE-2023-3696 Overview

CVE-2023-3696 is a prototype pollution vulnerability in the automattic/mongoose MongoDB object modeling library for Node.js. The flaw affects versions prior to 7.3.4 and resides in the document initialization logic. Attackers can inject properties into Object.prototype by supplying crafted keys such as __proto__ or constructor during document initialization. This vulnerability is tracked under CWE-1321: Improperly Controlled Modification of Object Prototype Attributes. Exploitation can lead to remote code execution, denial of service, or authentication bypass depending on how downstream application logic consumes polluted object properties.

Critical Impact

Network-reachable applications using Mongoose for document initialization with attacker-controlled input can suffer prototype pollution, leading to remote code execution and full compromise of confidentiality, integrity, and availability.

Affected Products

• automattic/mongoose versions prior to 7.3.4
• Node.js applications using Mongoose for MongoDB object modeling
• Downstream packages and services that pass untrusted input through Mongoose document initialization

Discovery Timeline

• 2023-07-17 - CVE-2023-3696 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-3696

Vulnerability Analysis

The vulnerability resides in the _init function within lib/document.js of Mongoose. During document initialization, the function iterates over keys from an input object and assigns corresponding values without filtering reserved JavaScript property names. When an attacker supplies keys such as __proto__ or constructor, the assignment propagates into Object.prototype, affecting every object in the running Node.js process.

Protocol pollution attacks against Node.js applications commonly lead to denial of service, security control bypass, or remote code execution when polluted properties intersect with template engines, child process spawning, or deserialization routines. The Mongoose initialization path is reachable through standard query result hydration and document construction APIs used in typical web applications.

Root Cause

The root cause is missing validation of object keys during document field initialization. The pre-patch _init function trusts all keys from the source object and proceeds to set schema paths without rejecting reserved prototype-modifying identifiers.

Attack Vector

The attack vector is network-based, requires no privileges, and needs no user interaction. An attacker submits a JSON payload containing __proto__ or constructor keys to any endpoint that constructs a Mongoose document from untrusted input. Once the prototype is polluted, the impact extends across the entire Node.js process lifetime.

// Security patch in lib/document.js - fix: avoid prototype pollution on init
function _init(index) {
    i = keys[index];
    // avoid prototype pollution
    if (i === '__proto__' || i === 'constructor') {
      return;
    }
    path = prefix + i;
    schemaType = docSchema.path(path);
}
// Source: https://github.com/automattic/mongoose/commit/305ce4ff789261df7e3f6e72363d0703e025f80d

The patch in commit 305ce4f adds an explicit check that returns early when the key matches __proto__ or constructor, preventing assignment to prototype-modifying property names.

Detection Methods for CVE-2023-3696

Indicators of Compromise

• HTTP request bodies or query parameters containing literal __proto__, constructor, or prototype keys submitted to Node.js endpoints
• Unexpected new properties appearing on built-in objects at runtime, such as enumeration of Object.prototype returning attacker-controlled values
• Anomalous Node.js process behavior including unexpected child process execution following document save or hydration calls
• Application errors referencing Mongoose document initialization paths after processing untrusted JSON input

Detection Strategies

• Inventory all Node.js services and identify Mongoose versions in package-lock.json and yarn.lock files to flag instances below 7.3.4
• Add runtime assertions that check Object.prototype for unexpected enumerable properties at startup and after request handling
• Deploy web application firewall rules that inspect JSON payloads for reserved property names like __proto__ and constructor
• Review application logs for Mongoose schema validation errors that correlate with malformed or unusual key names

Monitoring Recommendations

• Forward Node.js application logs and HTTP request metadata to a centralized analytics platform for prototype pollution pattern matching
• Alert on outbound connections or process spawns originating from Node.js workers shortly after document creation operations
• Track dependency drift through software composition analysis to maintain visibility into vulnerable Mongoose versions across services

How to Mitigate CVE-2023-3696

Immediate Actions Required

• Upgrade mongoose to version 7.3.4 or later in all Node.js services and rebuild deployment artifacts
• Audit application code that passes user-supplied objects directly into Mongoose document constructors or Model.hydrate calls
• Add input validation middleware that rejects requests containing __proto__, constructor, or prototype keys before they reach the ORM layer
• Restart all Node.js processes after upgrading to clear any potentially polluted runtime state

Patch Information

The fix is available in Mongoose 7.3.4, applied via commit 305ce4ff789261df7e3f6e72363d0703e025f80d. Review the GitHub Mongoose Commit Change and the Huntr Bounty Report for technical context.

Workarounds

• Use Object.freeze(Object.prototype) at Node.js process startup to prevent any modification of the global prototype
• Run Node.js with the --disable-proto=delete flag to remove the __proto__ accessor from runtime objects
• Sanitize all incoming JSON payloads using a schema validator such as Ajv or Joi configured to strip unknown and reserved keys before passing data to Mongoose

# Configuration example: upgrade Mongoose and harden Node.js runtime
npm install mongoose@^7.3.4 --save

# Verify installed version
npm ls mongoose

# Launch Node.js with prototype protection
node --disable-proto=delete app.js