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OWASP top 10 API Security vulnerabilities – Mass Assignment
April 12, 2023
OWASP API Security Mass Assignment


Mass Assignment refers to the risk of insecurely handling user input in APIs, which can allow attackers to modify or manipulate data in unintended ways. This can occur when APIs do not properly validate or sanitize user input, or when APIs allow direct assignment of user input to object properties without proper authorization or validation.


Some common risks associated with Mass Assignments include:

  • Unauthorized modification or manipulation of data
  • Elevation of privileges by unauthorized parties
  • Compromise of user accounts
Attack Scenarios

Attack scenarios for cloud applications may include:

  • An attacker intercepts an API call and modifies the request to modify or manipulate data in unintended ways
  • An attacker exploits a vulnerability in the API to directly assign user input to object properties, bypassing authorization or validation checks
  • An attacker uses an API to send malicious input in an attempt to exploit vulnerabilities or inject malicious code
Vulnerable Sample Code

A vulnerable sample of code in Go lang might look like this:

type User struct {
  ID        int    `json:"id"`
  Email     string `json:"email"`
  Password  string `json:"password"`
  FirstName string `json:"first_name"`
  LastName  string `json:"last_name"`
  Role      string `json:"role"`

func updateUser(w http.ResponseWriter, r *http.Request) {
  // Get the user's ID from the request
  userID := r.Header.Get("X-User-ID")

  // Get the updated user data from the request body
  var user User
  err := json.NewDecoder(r.Body).Decode(&user)
  if err != nil {
    http.Error(w, "Error decoding request body", http.StatusBadRequest)

  // Update the user in the database
  err = database.UpdateUser(userID, user)
  if err != nil {
    http.Error(w, "Error updating user", http.StatusInternalServerError)

  // Return a success message to the user
  json.NewEncoder(w).Encode("User updated successfully")

In this example, the API call allows a user to update their own data in a database. However, the API directly assigns the user input from the request body to the properties of a User struct without any validation or authorization checks. An attacker could exploit this vulnerability by intercepting the API call and modifying the request body to update the user’s data in unintended ways, such as changing the user’s role or password.

Sample Attack

A sample attack payload using the curl command might look like this:

curl -H "X-User-ID: attacker_user_id" -d '{"email":"attacker@example.com",
"role":"admin"}' -X PUT http://api.example.com/updateuser

In this example, the attacker is using curl to send a PUT request to the API with a modified user ID in the request header and a modified request body that includes a new email, password, and role for the user. If the API is vulnerable to Mass Assignment, the attacker may be able to update the user’s data in unintended ways.

MITRE ATT&CK framework reference

Mass Assignment can be mapped to the Tactic: Privilege Escalation and the Techniques: Exploitation of Uncontrolled Linkage to a Third-party


  1. Whitelisting: Only allow specific properties to be modified by the API. This can be done by using a whitelist of properties that are allowed to be modified, and rejecting any request that includes properties that are not on the whitelist.
  2. Strong data validation: Ensure that all data sent to the API is valid and conforms to the expected format. This can be done by using input validation libraries or by manually validating the data.
  3. Access control: Limit the API’s access to specific users or roles. This can be done by using role-based access control (RBAC) or by using API keys.
  4. Use of ORM: Use Object-Relational Mapping (ORM) frameworks like Entity Framework, Hibernate etc. These frameworks automatically handle the mass assignment vulnerability by not allowing the modification of properties that are not explicitly included in the mapping.
  5. Logging and monitoring: Keep track of all API requests and responses and log any suspicious activity. This will help you detect any mass assignment attempts and take appropriate action.
  6. Secure the API endpoints: Use HTTPS to encrypt the communication between the client and the API, and also use authentication and authorization mechanisms to limit access to the API.

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