Prototype Pollution

Prototype Pollution

Definition:

Prototype Pollution is a vulnerability affecting JavaScript code where an attacker is able to manipulate the prototype of an object. Since almost all objects in JavaScript inherit from a prototype, this manipulation can lead to unintended side effects in application behavior, potentially resulting in a breach of security.

Key Points:

• JavaScript objects can have prototypes, which are essentially references to other objects.

• When you attempt to access a property that doesn't exist in the object, JavaScript will look up its prototype chain until it finds the property or reaches an object with a null prototype.

• If properties on an object’s prototype are unsafely manipulated, especially objects that are passed as configuration or user input, it can cause a ripple effect on all objects inheriting from that prototype.

Common Attack Vectors:

1. Denial of Service (DoS):

   • By polluting object prototypes with recursive or deeply nested properties, an attacker could cause a service to enter an infinite loop or consume excessive resources.

2. Remote Code Execution (RCE) and Bypassing Security Checks:

   • If a piece of software uses an object property to configure critical functionality or security checks, prototype pollution might be exploited to alter the logic.

3. Property Overwrite:

   • Overwriting properties could lead to application logic being bypassed or changed, potentially leading to unauthorized actions.

Example Attack:

Suppose an application merges a user-provided object with a default object using a naive recursive merge function:

javascript

function merge(target, source) {
    for (let key in source) {
        if (typeof source[key] === 'object') {
            target[key] = merge(target[key] || {}, source[key]);
        } else {
            target[key] = source[key];
        }
    }
    return target;
}

// Use merge function
let userSettings = getUserInput(); // this gets data from the user
let defaultSettings = { display: true, fontSize: "14px" };
let settings = merge(defaultSettings, userSettings);

An attacker can provide input like:

javascript

{
  "display": "visible",
  "__proto__": {
    "isAdmin": true
  }
}

This would alter the prototype of all objects, adding an isAdmin property set to true.

Mitigation:

1. Don't Directly Merge User Input:

   • Avoid merging objects derived from user input directly with application-level objects.

2. Sanitize Keys:

   • Before merging or processing user input, ensure keys like __proto__, prototype, and constructor are blocked or sanitized.

3. Use Safe Libraries:

   • If you need to merge objects or handle user input, use well-reviewed libraries that are known to be safe from Prototype Pollution, like Lodash's _.merge() after v4.17.12.

4. Object Freezing:

   • Use Object.freeze() to prevent modification of an object, though be careful as this can have performance implications.

5. Update and Patch:

   • Keep JavaScript libraries and environments updated. Prototype Pollution vulnerabilities have been discovered in popular libraries, and maintaining an updated stack helps in mitigating known vulnerabilities.

Detection:

1. Static Code Analysis:

   • Tools like ESLint with security plugins can help detect patterns in code that are vulnerable to prototype pollution.

2. Dynamic Analysis:

   • Use fuzzing tools or penetration testing suites like Burp Suite to test applications for potential vulnerabilities.

3. Dependency Scanners:

   • Tools like Snyk or Dependabot can scan project dependencies for known vulnerabilities, including prototype pollution.

Conclusion:

Prototype Pollution is a subtle but potentially dangerous vulnerability in JavaScript applications. It takes advantage of the language's prototypal inheritance mechanism to introduce or alter properties, leading to unexpected and often undesirable behaviors in application logic. Developers should be cautious when handling objects, especially when they originate from user input, and employ a combination of safe coding practices and regular security assessments to prevent this vulnerability.