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Fix Mass Assignment in Buffalo

Mass Assignment in Buffalo (Go) is a classic 'over-posting' vuln. It happens when you blindly pass `c.Bind` to a database model. An attacker injects extra JSON/Form keys to flip bits they shouldn't touch—like `is_admin` or `account_balance`. Stop trust-falling your request body into your DB.

The Vulnerable Pattern

func (v UsersResource) Update(c buffalo.Context) error {
  tx := c.Value("tx").(*pop.Connection)
  user := &models.User{}
  if err := tx.Find(user, c.Param("user_id")); err != nil {
    return c.Error(404, err)
  }

// VULNERABLE: Binds request body directly to the DB model.
// If User struct has ‘Admin bool json:"admin"’, an attacker sends {“admin”: true}.
if err := c.Bind(user); err != nil {
return err
}
return tx.Update(user)
}

The Secure Implementation

The exploit leverages Go's reflection-based binding. When you bind to the model directly, Buffalo populates any matching struct tags found in the request. If your User model contains sensitive fields like 'Admin' or 'Permissions' with JSON/schema tags, a malicious POST request will overwrite them. The fix is the 'Allowlist' pattern: define a dedicated 'Form' struct containing only the fields a user is permitted to change. Bind the request to this intermediate struct, then manually map the values to your database model.

type UserUpdateForm struct {
  Email string `json:"email"` 
  Bio   string `json:"bio"` 
}

func (v UsersResource) Update(c buffalo.Context) error {
tx := c.Value(“tx”).(*pop.Connection)
form := &UserUpdateForm{}


// SECURE: Bind to a DTO (Data Transfer Object) / Allowlist struct
if err := c.Bind(form); err != nil {
return err
}


user := &models.User{}
if err := tx.Find(user, c.Param(“user_id”)); err != nil {
return c.Error(404, err)
}


// Explicitly map allowed fields only
user.Email = form.Email
user.Bio = form.Bio


return tx.Update(user)
}

System Alert • ID: 2345
Target: Buffalo API
Potential Vulnerability

Your Buffalo API might be exposed to Mass Assignment

74% of Buffalo apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.

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