Null Pointer Dereference in Fiber with Basic Auth

A null pointer dereference in a Fiber application that uses Basic Authentication can occur when the authenticated user object or credentials are expected to be non-nil but are not properly validated before access. In Go, dereferencing a nil pointer results in a runtime panic, which may be surfaced to the client as a 500 Internal Server Error or, in some configurations, cause the server to crash or restart.

When Basic Auth is used, the typical flow involves parsing the Authorization header, decoding the base64-encoded credentials, and mapping them to a user or role. If this mapping is implemented with a data structure such as a map without a prior existence check, and the code proceeds to call methods or access fields on a nil value, a null pointer dereference is introduced. For example, if the authenticated identity is derived from a database or in-memory store and the record is missing, the handler might receive a nil user pointer. Without a guard clause, any subsequent call like user.Role() will trigger a panic.

This becomes a security-relevant issue because the panic can be triggered by unauthenticated or low-privilege attackers who supply valid Basic Auth credentials that do not map to an existing user, or by sending malformed credentials that result in a nil user resolution. The resulting 500 responses can be used for denial-of-service reconnaissance or to infer internal implementation details. Because middleBrick tests unauthenticated attack surfaces and scans for Input Validation and Authentication issues in parallel, such runtime panics would be surfaced as high-severity findings under the Authentication and Input Validation checks, with remediation guidance to enforce non-nil checks before dereferencing.

Additionally, if the panic occurs inside middleware that is conditionally applied based on auth state, it may bypass intended authorization logic, effectively exposing functionality that should be protected. The scanner’s Authentication and BOLA/IDOR checks are designed to detect missing validation and unsafe handling of identity, which includes scenarios where a nil user is accepted as an authenticated principal. Proper handling requires explicit validation of the resolved identity before any property access, ensuring the application does not rely on implicit guarantees about credential-to-user mapping.

To prevent null pointer dereference in Fiber when using Basic Authentication, always validate the resolved user object before accessing its fields or methods. This involves checking for nil after credential resolution and returning an appropriate 401 Unauthorized response when the user cannot be found or authenticated.

Below is a secure, syntactically correct example of Basic Auth handling in Fiber that avoids nil dereferences:

// User represents an authenticated user.
type User struct {
	Username string
	Password string // In practice, store a hashed password
	Role     string
}

// In-memory user store for illustration.
var users = map[string]User{
	"alice": {Username: "alice", Password: "securepassword123", Role: "admin"},
}

func BasicAuthMiddleware(c *fiber.Ctx) error {
	header := c.Get("Authorization")
	if len(header) == 0 || len(header) <= 6 || header[:6] != "Basic " {
		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "authorization header missing or invalid"})
	}
	encodedCreds := header[6:]
	decoded, err := base64.StdEncoding.DecodeString(encodedCreds)
	if err != nil {
		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid authorization header"})
	}
	creds := string(decoded)
	parts := strings.Split(creds, ":")
	if len(parts) != 2 {
		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "credentials must be in format username:password"})
	}
	username, password := parts[0], parts[1]
	user, exists := users[username]
	if !exists {
		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid credentials"})
	}
	if user.Password != password {
		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid credentials"})
	}
	// Set user in context for downstream handlers.
	c.Locals("user", user)
	return c.Next()
}

func ProtectedHandler(c *fiber.Ctx) error {
	user, ok := c.Locals("user").(User)
	if !ok {
		return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "internal error"})
	}
	if user.Role != "admin" {
		return c.Status(fiber.StatusForbidden).JSON(fiber.Map{"error": "insufficient permissions"})
	}
	return c.JSON(fiber.Map{"message": "admin access granted"})
}

In this example, the users map is checked with the two-value assignment user, exists := users[username]. If the key does not exist, exists is false and the handler returns a 401 before any method calls are made on user. The downstream handler retrieves the user from context with a type assertion and validates the Role field, avoiding any nil pointer operations. This pattern aligns with the remediation guidance provided by middleBrick, which highlights the importance of validating authentication results and ensuring that identity resolution cannot produce nil values that are later dereferenced.

For automated enforcement in pipelines, the middleBrick CLI can be used to scan endpoints and detect missing nil checks as part of the Input Validation and Authentication findings. The GitHub Action can fail builds when such patterns are detected in scanned API definitions or runtime behavior, while the MCP Server enables developers to run scans directly from their IDEs to catch these issues early.