Buffer Overflow in Fiber with Basic Auth

A buffer overflow occurs when a program writes more data to a buffer than it can hold, corrupting adjacent memory. In the Fiber web framework for Go, using HTTP Basic Authentication can inadvertently expose or amplify this risk when user-controlled credentials are handled without proper length validation. If a handler reads the Authorization header and copies the username or password into a fixed-size byte array, an attacker can supply a very long credential string that overflows the buffer. This can lead to memory corruption, unexpected behavior, or potential code execution depending on the runtime environment and how the application manages memory.

For example, consider a Fiber route that manually parses Basic Auth credentials by reading the header and splitting on the colon. If the developer assumes credentials are short and uses a small stack-allocated buffer, an oversized username or password can overflow that buffer. Even though Go provides bounds safety in many scenarios, unsafe operations using CGo or direct byte manipulation can reintroduce risks. The combination of Fiber’s fast request handling and the presence of Basic Auth means that any unchecked credential input becomes an attractive vector for crafting malicious payloads that exploit memory handling flaws.

During a black-box scan, middleBrick tests unauthenticated endpoints and checks how the service handles oversized headers. If the API accepts long Authorization header values without truncation, validation, or safe copying, the scan can identify this as a potential buffer overflow condition under the Input Validation and Unsafe Consumption checks. The scanner does not assume the presence of an exploitable overflow but flags the lack of input sanitization as a security risk that could contribute to memory-related vulnerabilities when paired with other factors.

To mitigate buffer overflow risks when using HTTP Basic Authentication in Fiber, always treat credentials as untrusted input and avoid fixed-size buffers. Use Go’s native string handling, which is generally safe, and validate length before processing. Do not copy credential strings into fixed-length byte arrays. Instead, parse the header using standard libraries and enforce reasonable length limits.

Below are two concrete code examples for a Fiber route handling Basic Auth safely.

// Safe Basic Auth parsing in Fiber with length checks
package main

import (
	"encoding/base64"
	"fmt"
	"strings"

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

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

	app.Get("/secure", func(c *fiber.Ctx) error {
		auth := c.Get("Authorization")
		if auth == "" {
			return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "authorization header required"})
		}

		const prefix = "Basic "
		if !strings.HasPrefix(auth, prefix) {
			return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid authorization header format"})
		}

		payload := auth[len(prefix):]
		decoded, err := base64.StdEncoding.DecodeString(payload)
		if err != nil {
			return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid base64 encoding"})
		}

		parts := strings.SplitN(string(decoded), ":", 2)
		if len(parts) != 2 {
			return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "credentials must contain a colon"})
		}

		username, password := parts[0], parts[1]
		if len(username) > 256 || len(password) > 256 {
			return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "credentials too long"})
		}

		// Proceed with authentication logic using username and password
		fmt.Fprintf(c.Locals("username"), username)
		return c.SendStatus(fiber.StatusOK)
	})

	app.Listen(":3000")
}

This example validates the presence and format of the Authorization header, decodes Base64 safely, splits credentials with a limit on splits, and enforces maximum lengths for username and password. These checks reduce the risk of memory corruption by ensuring that no unbounded copy occurs.

// Using middleware to enforce credential length limits globally
package main

import (
	"strings"

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

func basicAuthMiddleware() fiber.Handler {
	return func(c *fiber.Ctx) error {
		auth := c.Get("Authorization")
		if auth != "" {
			const prefix = "Basic "
			if strings.HasPrefix(auth, prefix) {
				payload := auth[len(prefix):]
				// Decode and validate length here if needed
				// For middleware, you might set context values or reject oversized headers
				if len(payload) > 4096 {
					return c.Status(fiber.StatusRequestHeaderFieldsTooLarge).JSON(fiber.Map{"error": "authorization header too large"})
				}
			}
		}
		return c.Next()
	}
}

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

	app.Get("/items", func(c *fiber.Ctx) error {
		// Handler logic
		return c.JSON(fiber.Map{"status": "ok"})
	})

	app.Listen(":3000")
}

The second example shows a middleware approach to reject overly long Authorization headers before they reach individual routes, adding a layer of defense. By combining these practices with regular security scans using tools like middleBrick, you can detect missing length checks and ensure that Basic Auth handling in Fiber does not introduce buffer overflow risks.