Insecure Deserialization in Fiber with Basic Auth

Insecure deserialization occurs when an application processes untrusted serialized objects in a way that allows an attacker to manipulate the object’s state or execute code during reconstruction. In the Go web framework Fiber, developers commonly use middleware for authentication. When Basic Auth is used without additional validation and the request body or headers are deserialized with unsafe patterns (e.g., using gob, xml, or json.Unmarshal on attacker-controlled input), the combination can expose the application to deserialization attacks even though credentials are present.

Consider a Fiber route that authenticates via Basic Auth and then deserializes JSON or XML payloads into Go structs. If the handler does not validate the content type, does not enforce strict schema checks, or uses generic interfaces like interface{} with json.Unmarshal, an attacker can craft a serialized object that triggers unexpected behavior when unmarshaled. For example, XML External Entity (XXE) attacks against XML deserialization can read local files, and gadget chains in JSON or gob can lead to remote code execution. Even with Basic Auth providing transport-layer identity, the deserialization path remains a separate attack surface: the attacker sends a valid Authorization header and a malicious payload, bypassing auth checks while exploiting unsafe deserialization.

In a black-box scan, middleBrick tests inputs that include serialized formats and inspects whether the application reflects errors or executes unintended logic. A finding may indicate that the endpoint processes untrusted data without verifying integrity or applying type constraints. Because Basic Auth does not protect the deserialization process itself, the presence of credentials does not mitigate risks such as object injection or prototype pollution in interpreted languages if the backend uses embedded scripting. MiddleBrick’s checks include input validation and unsafe consumption to detect these patterns, and the findings are mapped to the OWASP API Top 10 and relevant CWE entries.

Real-world examples include endpoints that accept serialized job messages or configuration blobs. If an attacker can poison the serialized data, they may escalate from a low-severity input validation issue to a high-severity remote code execution path. The risk is compounded when the endpoint also exposes administrative functionality or interacts with sensitive resources. middleBrick’s cross-referencing of OpenAPI specs with runtime behavior helps identify mismatches where deserialization endpoints are not explicitly constrained, even when Basic Auth is present.

To secure a Fiber application using Basic Auth while preventing insecure deserialization, apply strict input validation, avoid unsafe deserializers, and enforce explicit schemas. Below are concrete code examples that demonstrate a hardened approach.

1. Safe Basic Auth Setup in Fiber

Use middleware that validates credentials and attaches user context without relying on deserialized credentials from the request body.

// main.go
package main

import (
	"net/http"
	"strings"

	"github.com/gofiber/fiber/v2"
	"github.com/gofiber/fiber/v2/middleware/basicauth"
)

func main() {
	app := fiber.New()

	cfg := basicauth.Config{
		Users: map[string]string{
			"admin": "securePass123", // in production, use a hashed lookup or environment variable
		},
		Validator: func(user, pass string) bool {
			// Add additional checks such as rate limiting or MFA flags here
			return user == "admin" && pass == "securePass123"
		},
	}

	app.Use(basicauth.New(cfg))

	app.Get("/public", func(c *fiber.Ctx) error {
		return c.SendString("public endpoint")
	})

	app.Get("/secure", func(c *fiber.Ctx) error {
		user := c.Locals("basicauth_user").(string)
		return c.JSON(fiber.Map{"message": "authenticated", "user": user})
	})

	app.Listen(":3000")
}

2. Avoid Unsafe Deserialization; Use Explicit DTOs

Never unmarshal directly into interface{} or use formats like gob/xml without strict schema validation. Use strongly typed structs and validate content type.

// handlers.go
package main

import (
	"net/http"

	"github.com/gofiber/fiber/v2"
)

type DataRequest struct {
	Action string `json:"action" validate:"required,oneof=create read update delete"`
	ItemID string `json:"item_id" validate:"required,max=64"`
}

func SafeHandler(c *fiber.Ctx) error {
	// Ensure only JSON is accepted
	if c.Get("Content-Type") != "application/json" {
		return c.Status(fiber.StatusUnsupportedMediaType).JSON(fiber.Map{"error": "unsupported media type"})
	}

	var req DataRequest
	if err := c.BodyParser(&req); err != nil {
		return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": err.Error()})
	}

	// Business logic using req.Action and req.ItemID
	return c.JSON(fiber.Map{"received": req})
}

3. Reject Dangerous Content Types and Add Size Limits

Configure Fiber to reject XML and other formats that carry deserialization risks, and enforce payload size limits to reduce abuse surface.

// config.go
package main

import (
	"github.com/gofiber/fiber/v2"
	"github.com/gofiber/fiber/v2/middleware/limitreq"
)

func configureApp() *fiber.App {
	app := fiber.New(fiber.Config{
		DisableStartupMessage: true,
		BodyLimit:             "1M", // limit request body size
	})

	// Optionally, add rate limiting to mitigate brute-force and flooding
	app.Use(limitreq.New(limitreq.Config{
		Max:        10,
		Expiration: 10,
	}))

These steps ensure that Basic Auth handles authentication while the application avoids unsafe deserialization patterns. middleBrick scans can validate that such controls are in place and flag endpoints that still process generic or unchecked serialized inputs.