Mass Assignment Cheat Sheet

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Last revision (mm/dd/yy): 02/17/2016

Introduction

"Modern frameworks allow developers to automatically bind HTTP request parameters from both request query and body into model objects for ease of development and increased productivity. If the binder is not correctly configured to control which HTTP request parameters are bound to which model attributes, an attacker may be able to abuse the model binding process and set any other attributes that should not be exposed to user control. This binding is possible even if the model attributes do not appear in the web forms or API contracts." - Mass Assignment: Sensitive Field Exposure

Example

Suppose there is a form for editing a user's account information:

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

Here is the object that the form is binding to:

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

Here is the controller handling the request:

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

Here is the typical request:

  POST /addUser
  
  userid=bobbytables&password=hashedpass&email=bobby@tables.com

And here is the exploit:

  POST /addUser
  
  userid=bobbytables&password=hashedpass&email=bobby@tables.com&isAdmin=true


The attacker can exploit this if:

  • They can guess common sensitive fields
  • They have access to source code and review the models for sensitive fields

General Solutions

  • Whitelist the bindable, non-sensitive fields
  • Blacklist the non-bindable, sensitive fields
  • Use Data Transfer Objects (DTOs)

Alternative Names

Depending on the language/framework in question, this vulnerability can have several alternative names

  • Mass Assignment: Ruby on Rails, NodeJS
  • Autobinding: Spring MVC, ASP.NET MVC
  • Object injection: PHP


Languages & Frameworks

Spring MVC

Whitelisting

  @Controller
  public class UserController
  {
     @InitBinder
     public void initBinder(WebDataBinder binder, WebRequest request)
     {
        binder.setAllowedFields(["userid","password","email"]);
     }
  
     ...
  }

Reference

Blacklisting

  @Controller
  public class UserController
  {
     @InitBinder
     public void initBinder(WebDataBinder binder, WebRequest request)
     {
        binder.setDisallowedFields(["isAdmin"]);
     }
  
     ...
  }

Reference

NodeJS

Ruby On Rails

Django

ASP.NET

PHP Laravel + Eloquent

Whitelisting

  <?php
  
  namespace App;
  
  use Illuminate\Database\Eloquent\Model;
  
  class User extends Model
  {
     private $userid;
     private $password;
     private $email;
     private $isAdmin;
  
     protected $fillable = array('userid','password','email');
  
  }

Reference

Blacklisting

  <?php
  
  namespace App;
  
  use Illuminate\Database\Eloquent\Model;
  
  class User extends Model
  {
     private $userid;
     private $password;
     private $email;
     private $isAdmin;
  
     protected $guarded = array('isAdmin');
  
  }

Reference

Authors and Primary Editors

References and future reading

Other Cheatsheets