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Fix Mass Assignment in Go Fiber

Mass Assignment in Go Fiber occurs when an attacker exploits the `c.BodyParser()` method to overwrite sensitive internal fields. This happens because developers often bind raw JSON input directly into a database model struct. If your 'User' struct contains a field like 'IsAdmin' and you pass that struct to the parser, an attacker can simply include '"is_admin": true' in their request payload to elevate privileges.

The Vulnerable Pattern

type User struct {
    ID        uint   `json:"id"` 
    Username  string `json:"username"` 
    Role      string `json:"role"` // Sensitive field
}

// VULNERABLE ROUTE
app.Post(“/update-profile”, func(c *fiber.Ctx) error {
var user User
// Directly parsing input into the DB model
if err := c.BodyParser(&user); err != nil {
return c.Status(400).JSON(fiber.Map{“error”: “Bad Request”})
}


// Attacker sends {"username": "hacker", "role": "admin"}
// db.Save overwrites the role field in the database
db.Save(&user)
return c.JSON(user)


})

The Secure Implementation

The fix involves decoupling your database models from your API input structures. Use a Data Transfer Object (DTO) struct that only contains fields the user is permitted to modify. By binding the request to this DTO instead of the full User model, any extra fields sent by the attacker (like 'role' or 'is_admin') are ignored by the Fiber parser. Finally, perform updates using explicit field selection or mapping to ensure only the whitelisted data hits your persistence layer.

type UpdateProfileDTO struct {
    Username string `json:"username"` 
    // Role is omitted, making it impossible to bind via BodyParser
}

// SECURE ROUTE
app.Post(“/update-profile”, func(c *fiber.Ctx) error {
var input UpdateProfileDTO
if err := c.BodyParser(&input); err != nil {
return c.Status(400).JSON(fiber.Map{“error”: “Invalid input”})
}


userId := c.Locals("user_id")
// Explicitly update only the allowed fields
if err := db.Model(&User{}).Where("id = ?", userId).Update("username", input.Username).Error; err != nil {
    return c.Status(500).SendString("Internal Server Error")
}

return c.SendStatus(200)


})

System Alert • ID: 6157
Target: Go Fiber API
Potential Vulnerability

Your Go Fiber API might be exposed to Mass Assignment

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

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