Difference between revisions of "Mass Assignment Cheat Sheet"

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= Introduction =
 
= Introduction =
 
=== Definition ===
 
=== Definition ===
"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." - [http://www.hpenterprisesecurity.com/vulncat/en/vulncat/java/mass_assignment_sensitive_field_exposure.html Mass Assignment: Sensitive Field Exposure]
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Software frameworks sometime allow developers to automatically bind HTTP request parameters into program code variables or objects to make using that framework easier on developers. This can sometimes cause harm. Attackers can sometimes use this methodology to create new parameters that the developer never intended which in turn creates or overwrites new variable or objects in program code that was not intended. This is called a <i>mass assignment</i> vulnerability.
  
 
=== Alternative Names ===
 
=== Alternative Names ===

Revision as of 19:30, 20 February 2016

Cheatsheets-header.jpg

Last revision (mm/dd/yy): 02/20/2016

Introduction

Definition

Software frameworks sometime allow developers to automatically bind HTTP request parameters into program code variables or objects to make using that framework easier on developers. This can sometimes cause harm. Attackers can sometimes use this methodology to create new parameters that the developer never intended which in turn creates or overwrites new variable or objects in program code that was not intended. This is called a mass assignment vulnerability.

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

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


Exploitability

This functionality becomes exploitable when:

  • Attacker can guess common sensitive fields
  • Attacker has access to source code and can review the models for sensitive fields
  • The object with sensitive fields has an empty constructor


Case Studies

GitHub

In 2012, GitHub was hacked using mass assignment. A user was able to upload his public key to any organization and thus make any subsequent changes in their repositories. GitHub's Blog Post


Solutions

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


General Solutions

Data Transfer Objects (DTOs)

An architectural approach is to create Data Transfer Objects and avoid binding input directly to domain objects. Only the fields that are meant to be editable by the user are included in the DTO.

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


Language & Framework Specific Solutions

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 + Mongoose

Reference

Ruby On Rails

Reference

Django

Reference

ASP.NET

Reference

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


Grails

Reference

Play

Reference

Jackson (JSON Object Mapper)

Reference Reference

GSON (JSON Object Mapper)

Reference Reference

JSON-Lib (JSON Object Mapper)

Reference

Flexjson (JSON Object Mapper)

Reference

Authors and Primary Editors

References and future reading

Other Cheatsheets