fiber: keyauth middleware requires a validator function

Error message

fiber: keyauth middleware requires a validator function

What it means

keyauth middleware authenticates requests by validating an extracted API key via a caller-supplied Validator function; without config there is no Validator to call. configDefault (keyauth/config.go:103-106) panics when New() is called with zero variadic arguments so the middleware cannot be mounted in a state that would accept every key (or reject every key) without explicit policy.

Source

Thrown at middleware/keyauth/config.go:105

}

// ConfigDefault is the default config
var ConfigDefault = Config{
	SuccessHandler: func(c fiber.Ctx) error {
		return c.Next()
	},
	ErrorHandler: func(c fiber.Ctx, _ error) error {
		return c.Status(fiber.StatusUnauthorized).SendString(ErrMissingOrMalformedAPIKey.Error())
	},
	Realm:     "Restricted",
	Extractor: extractors.FromAuthHeader("Bearer"),
}

// configDefault is a helper function to set default values
func configDefault(config ...Config) Config {
	// Return default config if nothing provided
	if len(config) < 1 {
		panic("fiber: keyauth middleware requires a validator function")
	}
	cfg := config[0]

	// Require a validator function
	if cfg.Validator == nil {
		panic("fiber: keyauth middleware requires a validator function")
	}

	// Set default values
	if cfg.Extractor.Extract == nil {
		cfg.Extractor = ConfigDefault.Extractor
	}
	if cfg.Realm == "" {
		cfg.Realm = ConfigDefault.Realm
	}
	if cfg.SuccessHandler == nil {
		cfg.SuccessHandler = ConfigDefault.SuccessHandler
	}

View on GitHub (pinned to 9a4c7e57fe)

Solutions

Pass a Config with a Validator: keyauth.New(keyauth.Config{Validator: func(c fiber.Ctx, key string) (bool, error) { ... }}).
If you are not ready to validate keys, remove the middleware from the router until the validator is implemented.

Example fix

// before
app.Use(keyauth.New())

// after
app.Use(keyauth.New(keyauth.Config{
    Validator: func(c fiber.Ctx, key string) (bool, error) {
        return key == os.Getenv("API_KEY"), nil
    },
}))

Defensive patterns

Strategy: type-guard

Validate before calling

func requireKeyAuthConfig(cfg ...keyauth.Config) (keyauth.Config, error) {
    if len(cfg) == 0 {
        return keyauth.Config{}, errors.New("keyauth.New requires a Config with a Validator")
    }
    return cfg[0], nil
}

c, err := requireKeyAuthConfig()
if err != nil { log.Fatal(err) }

Type guard

// Type guard: ensure the variadic config is present before calling New.
func hasKeyAuthConfig(cfg ...keyauth.Config) bool {
    return len(cfg) > 0
}

Prevention

Never call keyauth.New() with no arguments.
Wire the Validator before adding the middleware to the router.

When it happens

Trigger: Calling keyauth.New() with no arguments — keyauth.New() instead of keyauth.New(keyauth.Config{Validator: ...}).

Common situations: Refactoring that temporarily removes the config argument, or scaffolding code that wires the middleware before implementing the validator.

Related errors

AI-assisted analysis of gofiber/fiber@9a4c7e57fe (2026-08-04). Data as JSON: /data/errors/7b7ccc353cce8004.json. Report an issue: GitHub.