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API6:2019 - Mass Assignment

Threat agents/Attack vectors Security Weakness Impacts
API Specific : Exploitability 2 Prevalence 2 : Detectability 2 Technical 2 : Business Specific
Exploitation usually requires an understanding of the business logic, objects' relations, and the API structure. Exploitation of mass assignment is easier in APIs, since by design they expose the underlying implementation of the application along with the properties’ names. Modern frameworks encourage developers to use functions that automatically bind input from the client into code variables and internal objects. Attackers can use this methodology to update or overwrite sensitive object’s properties that the developers never intended to expose. Exploitation may lead to privilege escalation, data tampering, bypass of security mechanisms, and more.

Is the API Vulnerable?

Objects in modern applications might contain many properties. Some of these properties should be updated directly by the client (e.g., user.first_name or user.address) and some of them should not (e.g., user.is_vip flag).

An API endpoint is vulnerable if it automatically converts client parameters into internal object properties, without considering the sensitivity and the exposure level of these properties. This could allow an attacker to update object properties that they should not have access to.

Examples for sensitive properties:

  • Permission-related properties: user.is_admin, user.is_vip should only be set by admins.
  • Process-dependent properties: should only be set internally after payment verification.
  • Internal properties: article.created_time should only be set internally by the application.

Example Attack Scenarios

Scenario #1

A ride sharing application provides a user the option to edit basic information for their profile. During this process, an API call is sent to PUT /api/v1/users/me with the following legitimate JSON object:


The request GET /api/v1/users/me includes an additional credit_balance property:


The attacker replays the first request with the following payload:


Since the endpoint is vulnerable to mass assignment, the attacker receives credits without paying.

Scenario #2

A video sharing portal allows users to upload content and download content in different formats. An attacker who explores the API found that the endpoint GET /api/v1/videos/{video_id}/meta_data returns a JSON object with the video’s properties. One of the properties is "mp4_conversion_params":"-v codec h264", which indicates that the application uses a shell command to convert the video.

The attacker also found the endpoint POST /api/v1/videos/new is vulnerable to mass assignment and allows the client to set any property of the video object. The attacker sets a malicious value as follows: "mp4_conversion_params":"-v codec h264 && format C:/". This value will cause a shell command injection once the attacker downloads the video as MP4.

How To Prevent

  • If possible, avoid using functions that automatically bind a client’s input into code variables or internal objects.
  • Whitelist only the properties that should be updated by the client.
  • Use built-in features to blacklist properties that should not be accessed by clients.
  • If applicable, explicitly define and enforce schemas for the input data payloads.