Mass Assignment Exploit in Buffalo (Go)

Mass assignment occurs when a Go handler binds HTTP request parameters directly to a data model without filtering which fields can be set. In the Buffalo framework, this commonly happens through form values, query parameters, or JSON payloads passed to model binders. Because Buffalo encourages rapid development with conventions, developers may use helpers like models.Bind(model) or c.Param(&model) that map all incoming keys to struct fields. If the struct contains sensitive or mutable fields such as Role, Confirmed, or Permissions, an attacker can supply these in the request and change them unintentionally.

Consider a user profile update endpoint where the handler binds request data to a User model:

type User struct {
  ID        uuid.UUID `json:"id"`
  Email     string    `json:"email"`
  Role      string    `json:"role"`
  CreatedAt time.Time `json:"created_at"`
}

If the handler uses c.Bind(&user) without a permit-list, a PATCH request with {"role": "admin"} can elevate privileges. This maps directly into findings from the BOLA/IDOR and BFLA/Privilege Escalation checks in middleBrick, which test for missing authorization on object-level resources and excessive agency. The scanner also flags unauthenticated LLM endpoint exposure if such endpoints are reachable without authentication, and output scanning may detect role-related data leakage in responses. These checks align with OWASP API Top 10 A01:2023 broken object level authorization and A07:2021 identification and authentication failures.

Real-world attack patterns mirror these mechanics. For example, an attacker iterates over common field names like is_admin, role, verified, and subscription_tier to discover which fields are writable. middleBrick’s active prompt injection testing does not apply here, but the scanner’s inventory management and property authorization checks highlight missing constraints on model binding. Because Buffalo does not enforce field filtering by default, developers must explicitly define which fields are allowed to be updated, especially for sensitive attributes.

Remediation centers on avoiding automatic binding to models and instead using explicit allow-lists. In Buffalo, replace broad bind calls with selective assignment or validated input mapping. Below are concrete, working Go examples demonstrating secure handling for a user profile update.

Insecure pattern (vulnerable)

// User model with sensitive fields
type User struct {
  ID        uuid.UUID `json:"id"`
  Email     string    `json:"email"`
  Role      string    `json:"role"`
  Password  string    `json:"password"`
}

// Vulnerable handler
func UsersUpdate(c buffalo.Context) error {
  user := &User{}
  if err := c.Bind(user); err != nil {
    return err
  }
  // Save user...
  return c.Render(200, r.JSON(user))
}

The above allows an attacker to modify Role and Password via crafted requests, triggering BFLA and data exposure findings in the middleBrick report.

Secure pattern (recommended)

// DTO for updates, only including allowed fields
type UserUpdateDTO struct {
  Email    *string `json:"email" validate:"required,email"`
  Name     *string `json:"name" validate:"required,max=255"`
}

// Secure handler
func UsersUpdate(c buffalo.Context) error {
  dto := &UserUpdateDTO{}
  if err := c.Bind(dto); err != nil {
    return err
  }

  // Fetch existing user from store
  var user User
  if err := c.Param(&user); err != nil {
    return err
  }

  // Apply only allowed fields
  if dto.Email != nil {
    user.Email = *dto.Email
  }
  if dto.Name != nil {
    user.Name = *dto.Name
  }

  // Save updated user...
  return c.Render(200, r.JSON(user))
}

This approach ensures only explicitly permitted fields are updated. The use of pointers in UserUpdateDTO allows distinguishing between omitted fields and fields set to zero values. For JSON APIs, prefer binding to an intermediate map and validating keys against an allow-list:

var input map[string]interface{}
if err := c.Bind(&input); err != nil {
  return err
}
allowed := map[string]bool{"email": true, "name": true}
for key := range input {
  if !allowed[key] {
    return c.Error(400, errors.New("invalid field: " + key))
  }
}

Additionally, apply server-side validation using a library like go-playground/validator and ensure sensitive fields are never bound from client input. middleBrick’s property authorization checks can verify that updates respect field-level permissions, and the inventory management checks confirm that only documented, expected fields are processed.