Mass Assignment Vulnerability: How It Works & 6 Defensive Measures

What Is a Mass Assignment Vulnerability? 

Mass assignment vulnerabilities occur when an application automatically assigns user input to model properties without proper filtering or validation. This allows attackers to modify object properties they shouldn't be able to access, such as changing a user's permissions, email, or password.

These vulnerabilities often occur in applications that use frameworks allowing mass assignment from request parameters. Without strict controls, attackers can supply unexpected parameters through common methods like POST requests, leading to unauthorized changes in the application's data.

This is part of a series of articles about OWASP Top 10

In this article:

• The Impact of a Mass Assignment Vulnerability
• How Mass Assignment Vulnerabilities Work 
• Example of a Mass Assignment Attack
• 6 Ways to Mitigate the Security Risks of Mass Assignment
  • 1. Allowlist Allowed Attributes‍
  • 2. Sanitize and Validate Input‍
  • 3. Keep Dependencies Up-to-Date‍
  • 4. Employ Strong Authentication and Authorization‍
  • 5. Use Role-Based Access Control (RBAC)

The Impact of a Mass Assignment Vulnerability

The impact can be severe, depending on the data an attacker can modify. It might lead to privilege escalation, data leakage, or full account takeover. This could result in significant financial losses, legal penalties, or damage to an organization's reputation.

By exploiting mass assignment vulnerabilities, attackers can bypass usual access controls, altering critical system settings or user data without detection. The scale of the attack often hinges on the application's data sensitivity and what the altered parameters control.

Tzvika Shneider

CEO, Pynt

Tzvika Shneider is a 20-year software security industry leader with a robust background in product and software management.

Tips from the expert

• Use allowlists for properties: Explicitly define the properties that can be modified through API requests, reducing the risk of unauthorized fields being altered.
• Implement server-side checks: Ensure the server validates the data before processing, rejecting any unexpected fields in incoming requests.
• Restrict JSON deserialization: Restrict JSON deserialization to predefined models, preventing attackers from injecting malicious or unintended properties.
• Employ input sanitization: Sanitize all input to strip out properties that are not explicitly allowed or that exceed defined data constraints.
• Audit API endpoints: Regularly audit API endpoints for potential mass assignment vulnerabilities, especially after introducing new features or making changes to data models.

How Mass Assignment Vulnerabilities Work 

Mass assignment vulnerabilities exploit how frameworks handle user input. When an application doesn’t differentiate between which parameters should and shouldn’t be modified directly by a user, it opens up a vector for attack. Once such a weak point is identified, the attacker crafts malicious HTTP requests that include parameters targeting these unprotected attributes.

For example, consider a scenario where an application uses an object to store user profile information. The attacker can send a POST request with additional, unanticipated parameters such as role=admin or status=disabled. If these parameters are not explicitly filtered out or validated against an allowlist, the application's backend logic might accept these as legitimate and alter the user object’s properties accordingly. 

Learn more in our detailed guide to business logic vulnerabilities

Thus, attackers can gain elevated privileges or disrupt services by manipulating key object attributes through seemingly innocuous user input. These attacks are made possible by the common practice of binding form or API input directly to data models. Without rigorous checks and balances, such as field-level validation, attackers can easily insert unauthorized data into these models. 

This is particularly dangerous when combined with other vulnerabilities or weak points within the application, such as insecure direct object references or insufficient logging and monitoring, which can mask or facilitate unauthorized access and modifications.

Related content: Read our guide to owasp top 10 cheat sheet

Example of a Mass Assignment Attack 

This example was adapted from the official OWASP cheat sheet. Let’s look at a scenario where a mass assignment vulnerability exists in a basic web application designed for editing user account information. 
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The application comprises a straightforward HTML form that captures a user's ID, password, and email address. This form interacts with a backend controller upon submission, binding user input to a corresponding User object.

The form is outlined as follows:

 <form>     
    <input name="userid" type="text">     
    <input name="password" type="text">     
    <input name="email" text="text">     
    <input type="submit">
  </form>

The User object, meant to receive the form data, looks like this:

public class User {  
	private String userid;   
    private String password;   
    private String email;   
    private boolean isAdmin;‍   
    // Getters and Setters
 }

The server-side handling of the form submission is managed by a controller mapped to /addUser and designed to process POST requests. Upon receiving a form submission, it invokes a service method to add the user data to the system:

@RequestMapping(value = "/addUser", method = RequestMethod.POST)
	public String submit(User user) {   
    	userService.add(user);   
        return "successPage";
     }

A typical, legitimate request to this endpoint might look like this:

POST /addUser
...
userid=johnsmith&password=hashedpass&email=johns@site.com

However, exploiting the mass assignment vulnerability involves manipulating the request to include an isAdmin parameter, which is not intended to be modified directly by end users through the form. By appending &isAdmin=true to the request, an attacker can alter the isAdmin property of the User object, granting themselves administrative privileges:

POST /addUser
...
userid=johnsmith&password=hashedpass&email=johns@site.com&isAdmin=true

This exploit demonstrates the risk associated with indiscriminately binding user input to model attributes, especially when sensitive properties like isAdmin are involved. Without adequate filtering or validation, this vulnerability can compromise the integrity of the application.

6 Ways to Mitigate the Security Risks of Mass Assignment 

Here are some of the measures that can be used to prevent mass assignment attacks.

1. Allowlist Allowed Attributes

Define which attributes can be safely exposed to mass assignment. Using an allowlist approach ensures only specified attributes can be updated through user input.

This method requires explicitly listing allowed parameters, making unintended data modifications less likely. Regularly review and update the allowlist to accommodate changes in the application’s functionality.

2. Sanitize and Validate Input

Sanitizing input involves stripping harmful data before it’s processed, whereas validation ensures the data meets specific criteria. Together, they can prevent attackers from submitting malicious or unexpected data through mass assignment.

Input should be sanitized to remove potential executable code or SQL commands. Validation rules, like checking for allowable values or correct data types, further reduce the risk of unauthorized modifications.

3. Keep Dependencies Up-to-Date

Applications often rely on external libraries or frameworks that might contain vulnerabilities, including those leading to mass assignment issues. Regularly update these dependencies to ensure that known vulnerabilities are patched.

Subscribe to notifications from dependency providers and security bulletins to stay informed about relevant security updates. Automated tools can help identify outdated dependencies in a project, simplifying the update process.

4. Employ Strong Authentication and Authorization

Authentication mechanisms ensure that users are who they claim to be, while authorization checks confirm they have permission for the requested actions. Ensuring both are in place can significantly reduce the risk of mass assignment vulnerabilities being exploited.
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Authorization checks should occur at every stage of data processing, especially before sensitive operations like data updates. This prevents unauthorized users from exploiting mass assignment vulnerabilities to access or modify data.

5. Use Role-Based Access Control (RBAC)

RBAC ensures users can only interact with data and actions appropriate for their role. This helps mitigate risks by limiting what authenticated users can modify, even if a mass assignment vulnerability exists.

RBAC requires defining roles and permissions carefully, ensuring they align with the principle of least privilege. This means users get only the access necessary for their role, reducing the potential impact of a mass assignment vulnerability.

6. Monitor and Log Activities

Monitoring and logging access and modification of sensitive data help detect unauthorized attempts to exploit mass assignment vulnerabilities. Automated alerts can notify administrators of suspicious activities, enabling rapid response to potential breaches.

Logs should capture sufficient detail to understand the nature of each attempt, including who made the request and what was attempted. Regular analysis of logs can also help identify patterns indicating unaddressed vulnerabilities.

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