mass assignment vulnerability- Web Application Security

Mass Assignment Vulnerabilities

Web application security is of critical importance in today’s digital landscape. Developers need to be aware of various vulnerabilities that malicious actors can exploit. One such critical vulnerability is “Mass Assignment Vulnerability,” a commonly overlooked security flaw in web applications. This article aims to shed light on the nature of this threat, its implications, and how developers can prevent it. Additionally, we’ll provide an example of vulnerable PHP code to help illustrate the concept.

Table of Contents

What is Mass Assignment Vulnerability?

Mass Assignment Vulnerability occurs when a web application allows users to submit a more extensive set of data than is intended or safe. Typically, this vulnerability arises due to careless handling of user inputs and inadequate validation and sanitization. In some cases, the developer might intend to allow specific data updates using a particular request but unintentionally enables the modification of sensitive or protected fields.

The potential consequences of this vulnerability can be severe, as attackers can exploit it to alter data, escalate privileges, or even gain unauthorized access to critical system components. Therefore, it is crucial for developers to identify and mitigate this vulnerability in their applications.

Example of Vulnerable PHP Code

Consider a hypothetical scenario with a PHP-based web application that allows users to update their profile information, such as name, email, and account type. The developer has created a simple class to handle user updates:

How to Prevent Mass Assignment
Figure 1: Vulnerable Code

In this code snippet, the developer fetches a JSON object from the POST body and directly uses it to update the user’s profile without proper validation. An attacker could manipulate the POST request, including additional data fields like ‘isAdmin’ or ‘isActivated,’ which should never be allowed to be modified by regular users.  The above code also implements an auto-mapper functionality that populates the User class properties with the corresponding values found in the $userObject object, which is user-controlled input. In figure 2 an example of a HTTP request with the expected JSON input object can be seen. In figure 3, a request showing how the “isAdmin” property can be updated is shown.

Secure Code to Prevent Hackers
Figure 2: Expected Input
Figure 3: Malicious Input

To prevent a user from populating class properties that should be inaccessible, an allow list should be implemented as seen in the code below. The highlighted code defines an allow list containing the “name” and “email” properties.  When the auto-mapper assigns values to the class properties, the current property is checked against the allow list.

Web Application Security
Figure 4: Less Vulnerable Code

Mitigation Techniques

To prevent Mass Assignment Vulnerabilities, developers can implement the following best practices:

  1. Allowlist Input: Only update specific, intended fields. Create a list of allowed fields and update only those explicitly mentioned in the whitelist.
  2. Blocklist Rejected Fields: If the application requires users to update almost all fields except a few, developers can create a blocklist of disallowed fields.
  3. Input Validation: Always validate and sanitize user inputs to ensure they adhere to the expected format and are free from malicious data.
  4. Role-based Access Control (RBAC): Implement RBAC to define different user roles and permissions, ensuring that sensitive fields are only accessible to authorized personnel.
  5. Use Frameworks with Built-in Protection: Utilize web application frameworks with built-in security mechanisms to prevent Mass Assignment Vulnerabilities.


Mass Assignment Vulnerabilities pose a significant threat to web applications, potentially allowing attackers to manipulate critical data and gain unauthorized access. Developers must be vigilant and implement robust input validation and access control mechanisms to prevent this vulnerability. Developers can create more secure web applications and protect sensitive user data from potential breaches by adhering to best practices, testing and staying updated on the latest security measures.

Picture of Anthony Cole, Sr. Penetration Tester at Redbot Security

Anthony Cole, Sr. Penetration Tester at Redbot Security

Anthony Cole is a Sr. Security Consultant with over 22 years of experience in information technology, IT security and software development. Anthony is fully GIAC certified in all facets of information security, enabling him to facilitate successful outcomes for customers. Anthony’s vast knowledge of both offensive and defensive security ensures that Redbot Security’s customers will receive the best service in the industry.

Anthony is Redbot Security’s AppSec SME and formerly a Sr. Level Application Penetration Testing Engineer for NetSpi and Presidio as well as Blutique LLC’s Chief Technical Officer and Sr. Application Developer.

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