Mass Assignment in Buffalo

Mass assignment vulnerabilities in Buffalo applications typically emerge when user-supplied data is directly bound to model structures without proper field filtering. In Buffalo's convention-over-configuration approach, this often happens through the bind method in controllers or when using pop for database operations.

The most common pattern involves controller actions that accept JSON payloads and automatically bind them to Buffalo models. Consider this vulnerable endpoint:

func CreateUser(c buffalo.Context) error {
    user := &models.User{}
    if err := c.Bind(user); err != nil {
        return err
    }
    
    tx := c.Value("tx").(*pop.Connection)
    err := tx.Create(user)
    if err != nil {
        return c.Error(500, err)
    }
    
    return c.Render(200, r.JSON(user))
}

An attacker can exploit this by sending additional fields that get assigned to the user model:

{
  "email": "victim@example.com",
  "password": "secure123",
  "role": "admin",
  "is_active": true
}

If the User model includes fields like Role, IsActive, or other administrative properties, these will be overwritten without validation. This is particularly dangerous in Buffalo applications because the framework's automatic binding makes it easy to overlook field whitelisting.

Another Buffalo-specific manifestation occurs in update operations where partial updates are allowed:

func UpdateUser(c buffalo.Context) error {
    user := &models.User{}
    if err := c.Bind(user); err != nil {
        return err
    }
    
    tx := c.Value("tx").(*pop.Connection)
    err := tx.Update(user)
    if err != nil {
        return c.Error(500, err)
    }
    
    return c.Render(200, r.JSON(user))
}

Without proper field filtering, attackers can modify any user attribute, including sensitive fields like Admin, , or Permissions.

Buffalo's pop library can also introduce mass assignment risks when using Save or Update operations on models that include sensitive fields. The framework's convention of using struct tags for database mapping means that any exported field can potentially be modified if not explicitly protected.

Detecting mass assignment vulnerabilities in Buffalo applications requires examining both the code patterns and the data flow. Using middleBrick's CLI tool, you can scan your Buffalo API endpoints for these specific vulnerabilities:

middlebrick scan http://localhost:3000/api/users
middlebrick scan --output json http://localhost:3000/api/users > report.json

middleBrick specifically tests for mass assignment by sending crafted payloads with additional fields that should not be modifiable. For Buffalo applications, it checks:

• Whether sensitive fields like role, admin, can be modified through API endpoints
• If IDOR (Insecure Direct Object References) combined with mass assignment allows privilege escalation
• Whether update operations properly validate field permissions
• If create operations allow setting administrative flags

The scanner also examines your Buffalo models to identify fields that should be protected, comparing them against the actual API behavior. For example, if your User model has an Admin field but the API allows setting it, middleBrick will flag this as a critical vulnerability.

Manual detection in Buffalo code involves searching for these patterns:

# Look for bind operations without field filtering
grep -r 'c.Bind(' actions/ | grep -v 'whitelisted'

Also examine your model definitions for sensitive fields that should not be user-modifiable:

type User struct {
    ID        uuid.UUID  `json:"id" db:"id"`
    Email     string     `json:"email" db:"email"`
    Password  string     `json:"password" db:"password"`
    Role      string     `json:"role" db:"role"`          // Should be admin-only
    IsActive  bool       `json:"is_active" db:"is_active"` // Should be admin-only
    CreatedAt time.Time  `json:"created_at" db:"created_at"`
    UpdatedAt time.Time  `json:"updated_at" db:"updated_at"`
}

Pay special attention to fields with administrative significance, timestamps that shouldn't be modified, and any flags that control access or functionality.

Buffalo provides several native approaches to prevent mass assignment vulnerabilities. The most straightforward is using the Bind method with a whitelist:

func CreateUser(c buffalo.Context) error {
    user := &models.User{}
    
    // Only allow specific fields to be bound
    allowed := map[string]interface{}{}
    if err := c.Bind(allowed); err != nil {
        return err
    }
    
    // Manually assign allowed fields
    user.Email = allowed["email"].(string)
    user.Password = allowed["password"].(string)
    
    tx := c.Value("tx").(*pop.Connection)
    err := tx.Create(user)
    if err != nil {
        return c.Error(500, err)
    }
    
    return c.Render(200, r.JSON(user))
}

For update operations, use field-specific updates:

func UpdateUser(c buffalo.Context) error {
    userID, err := uuid.FromString(c.Param("id"))
    if err != nil {
        return c.Error(400, err)
    }
    
    tx := c.Value("tx").(*pop.Connection)
    user := &models.User{}
    err = tx.Find(user, userID)
    if err != nil {
        return c.Error(404, err)
    }
    
    updates := map[string]interface{}{}
    if err := c.Bind(updates); err != nil {
        return err
    }
    
    // Only allow specific fields to be updated
    if email, ok := updates["email"].(string); ok {
        user.Email = email
    }
    if password, ok := updates["password"].(string); ok {
        user.Password = password
    }
    
    err = tx.Update(user)
    if err != nil {
        return c.Error(500, err)
    }
    
    return c.Render(200, r.JSON(user))
}

Buffalo's pop library also supports model-level field permissions through struct tags:

type User struct {
    ID        uuid.UUID  `json:"id" db:"id"`
    Email     string     `json:"email" db:"email"`
    Password  string     `json:"password" db:"password"`
    Role      string     `json:"-" db:"role"`          // Exclude from JSON binding
    IsActive  bool       `json:"-" db:"is_active"`     // Exclude from JSON binding
    CreatedAt time.Time  `json:"created_at" db:"created_at"`
    UpdatedAt time.Time  `json:"updated_at" db:"updated_at"`
}

The json:"-" tag prevents fields from being included in JSON binding operations, effectively protecting them from mass assignment through API endpoints.

For more complex scenarios, create dedicated request models that define exactly what can be modified:

type UserCreateRequest struct {
    Email    string `json:"email" db:"email"`
    Password string `json:"password" db:"password"`
}

type UserUpdateRequest struct {
    Email    string `json:"email" db:"email"`
    Password string `json:"password" db:"password"`
}

func CreateUser(c buffalo.Context) error {
    req := &UserCreateRequest{}
    if err := c.Bind(req); err != nil {
        return err
    }
    
    user := &models.User{
        Email:    req.Email,
        Password: req.Password,
    }
    
    tx := c.Value("tx").(*pop.Connection)
    err := tx.Create(user)
    if err != nil {
        return c.Error(500, err)
    }
    
    return c.Render(200, r.JSON(user))
}

This approach ensures that only explicitly defined fields can be modified through each API endpoint, eliminating the risk of unintended field assignment.