Type Confusion in Fiber

Type confusion in Fiber applications occurs when the framework's dynamic type handling is exploited to bypass validation or access unauthorized data. This vulnerability is particularly dangerous in Go-based web frameworks like Fiber because of Go's interface{} type system and Fiber's flexible request binding mechanisms.

The most common manifestation appears in parameter binding. Consider this typical Fiber handler:

func getUser(c *fiber.Ctx) error {
    var params struct {
        ID string `json:"id"`
    }
    
    if err := c.Bind(&params); err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": err.Error()})
    }
    
    // Direct use without validation
    userID := params.ID
    user := database.GetUser(userID)
    return c.JSON(user)
}

The vulnerability emerges when attackers manipulate the request body to include type coercion. If the backend database expects a specific type (like an integer ID) but receives a string, type confusion can lead to SQL injection or NoSQL injection patterns. For example, sending {"id": "1 OR 1=1"} might bypass authorization checks.

Another Fiber-specific vector involves middleware type assertions. Fiber's middleware chain processes requests through multiple layers, and improper type assertions can create confusion:

func authMiddleware(c *fiber.Ctx) error {
    user := c.Locals("user")
    
    // Unsafe type assertion
    if user.(int) != 0 {
        return c.Next()
    }
    return c.Status(fiber.StatusUnauthorized).SendString("Unauthorized")
}

func adminRoute(c *fiber.Ctx) error {
    // This might receive a string, int, or other type
    userID := c.Locals("user")
    
    // Type confusion vulnerability
    if userID == "admin" {
        return c.JSON(adminData)
    }
    return c.JSON(userData)
}

The combination of Go's interface{} system and Fiber's dynamic binding creates scenarios where type confusion can bypass authorization checks, leading to privilege escalation or data exposure.

Detecting type confusion in Fiber applications requires examining both the code structure and runtime behavior. The vulnerability typically appears in three key areas: parameter binding, middleware type assertions, and database query construction.

Static analysis should focus on these patterns:

# Look for unsafe type assertions
grep -r "\.\(\w\+\)" . --include="*.go"

# Find direct interface{} usage without validation
grep -r "interface\{\}" . --include="*.go"

# Identify unvalidated parameter binding
find . -name "*.go" -exec grep -l "Bind(" {} \; | xargs grep -n "c\.Bind"

Dynamic detection with middleBrick specifically targets Fiber applications by scanning the unauthenticated attack surface for type confusion patterns. The scanner tests parameter binding by sending malformed JSON with type coercion attempts:

{
    "id": "0 OR 1=1",
    "user_id": 123,
    "role": "admin",
    "permissions": [true, false, true]
}

middleBrick's black-box scanning methodology is particularly effective for Fiber applications because it doesn't require source code access. The scanner sends requests with various type manipulation payloads and analyzes the responses for signs of type confusion exploitation, such as inconsistent error messages, unexpected data exposure, or authorization bypass.

The scanner also examines Fiber's middleware chain by testing how different request types propagate through the application layers, identifying where unsafe type assertions might allow unauthorized access.

Remediating type confusion in Fiber applications requires a defense-in-depth approach that validates types at every boundary. The first layer is strict parameter validation using Fiber's built-in validation features:

type UserParams struct {
    ID int    `json:"id" validate:"required,number"`
    Role string `json:"role" validate:"omitempty,alpha"`
}

func getUser(c *fiber.Ctx) error {
    var params UserParams
    
    if err := c.Bind(&params); err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "Invalid parameters"})
    }
    
    // Validate using go-playground/validator
    validate := validator.New()
    if err := validate.Struct(params); err != nil {
        return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": err.Error()})
    }
    
    // Safe type usage
    userID := params.ID
    user := database.GetUserByID(userID)
    return c.JSON(user)
}

For middleware, implement type-safe patterns that avoid interface{} assertions:

type User struct {
    ID       int    `json:"id"`
    Username string `json:"username"`
    Role     string `json:"role"`
}

func authMiddleware(c *fiber.Ctx) error {
    // Parse token and validate
    token := c.Get("Authorization")
    if token == "" {
        return c.Status(fiber.StatusUnauthorized).SendString("Missing token")
    }
    
    user, err := authenticateUser(token)
    if err != nil {
        return c.Status(fiber.StatusUnauthorized).SendString("Invalid token")
    }
    
    // Store typed user object
    c.Locals("user", user)
    return c.Next()
}

func adminRoute(c *fiber.Ctx) error {
    user := c.Locals("user")
    
    // Safe type assertion with validation
    typedUser, ok := user.(*User)
    if !ok || typedUser.Role != "admin" {
        return c.Status(fiber.StatusForbidden).SendString("Access denied")
    }
    
    return c.JSON(adminData)
}

Database queries should use parameterized statements to prevent type confusion exploitation:

func getUserByID(c *fiber.Ctx) error {
    var params struct {
        ID int `json:"id"`
    }
    
    if err := c.Bind(&params); err != nil {
        return c.Status(fiber.StatusBadRequest).SendString("Invalid ID format")
    }
    
    // Use parameterized queries
    var user User
    row := database.QueryRow("SELECT * FROM users WHERE id = ?", params.ID)
    
    if err := row.Scan(&user.ID, &user.Name, &user.Email); err != nil {
        return c.Status(fiber.StatusNotFound).SendString("User not found")
    }
    
    return c.JSON(user)
}

Implementing these patterns throughout your Fiber application eliminates type confusion vulnerabilities by ensuring type safety at every interface boundary.