Credential Stuffing in Buffalo

In Go-based web services using the Buffalo framework, credential stuffing attacks often target authentication endpoints that lack rate limiting or account lockout mechanisms. Attackers automate login attempts using breached credential lists, exploiting endpoints like /auth/login that validate username and password without throttling. Buffalo’s default handlers, when built with buffalo generate auth, create a AuthResource struct with a Login method that typically calls c.Bind(&user) to extract credentials and tx.Where(&models.User{Email: user.Email}).First(&foundUser) to check existence. If the application does not enforce delays, CAPTCHAs, or attempt limits per IP or account, attackers can send hundreds of requests per second using tools like Sentry MBA or custom scripts.

A common vulnerable pattern appears in Buffalo handlers where authentication logic resides directly in the action without middleware interception. For example, a handler might process login as follows: validate input, query the database for a matching email, compare bcrypt hashes, and set a session cookie — all within a single HTTP request cycle. Without intervening controls, this allows rapid credential validation. Attackers may also enumerate valid usernames by observing timing differences or error messages (e.g., "invalid password" vs "user not found"), which Buffalo’s default Authorize middleware does not inherently prevent if customized poorly.

These attacks succeed when Buffalo applications deploy authentication logic without integrating protective layers such as rate-limiting middleware, account lockout after failed attempts, or anomaly detection. The absence of such controls turns the login endpoint into an oracle for credential validity, enabling large-scale account takeover attempts that evade detection due to distributed source IPs and low-frequency per-IP requests.

Detecting credential stuffing vulnerabilities in Buffalo applications requires observing whether authentication endpoints enforce request throttling or account-based limits. middleBrick identifies this risk during its unauthenticated black-box scan by probing login endpoints with rapid sequential requests and analyzing responses for absence of rate-limiting signals (e.g., missing 429 Too Many Requests status, lack of Retry-After header, or no delay in response timing under load). It also checks for informative error messages that could aid username enumeration — a precursor to credential stuffing.

For instance, when scanning a Buffalo-generated AuthResource.Login endpoint, middleBrick sends bursts of login attempts with varying credentials and monitors:

• Whether the application returns consistent timing regardless of credential validity (indicating lack of user enumeration protection)
• If successive requests from the same IP are not delayed or blocked after a threshold (e.g., 5 failed attempts)
• Whether responses leak information via status codes or body content (e.g., distinguishing 401 Unauthorized for bad password vs 404 Not Found for missing user)

These observations map to OWASP API4:2023 (Unrestricted Resource Consumption) and API7:2023 (Server Side Request Forgery) in contexts where login triggers external calls, but primarily reflect missing authentication throttling. middleBrick’s report highlights the endpoint, observed request rate tolerance, and any enumerative behavior, providing severity based on whether both rate limiting and username protection are absent.

Developers can replicate this check locally using Buffalo’s testing tools: simulate multiple login requests via http.Test and assert that status codes or timing do not reveal user existence or allow unlimited attempts.

To mitigate credential stuffing in Buffalo applications, implement rate limiting and authentication hardening using Buffalo’s middleware system and Go idioms. Since Buffalo does not include built-in brute-force protection, developers must add custom middleware or leverage libraries like github.com/ulule/limiter/v3 with github.com/gin-contrib/limiter adapters (compatible via Buffalo’s buffalo/middleware pattern).

First, create a rate-limiting middleware that tracks failed login attempts per IP and account. Example:

import (
	"github.com/gobuffalo/buffalo/middleware"
	"github.com/gobuffalo/buffalo"
	"golang.org/x/time/rate"
	"sync"
	"time"
)

var (
	limiterMap = make(map[string]*rate.Limiter)
	mu         sync.Mutex
)

func LoginRateLimiter(next buffalo.Handler) buffalo.Handler {
	return func(c buffalo.Context) error {
		if c.Request().URL.Path != "/auth/login" || c.Request().Method != "POST" {
			return next(c)
		}
		ip := c.Request().RemoteAddr
		mu.Lock()
		l, exists := limiterMap[ip]
		if !exists {
			l = rate.NewLimiter(5, 10) // 5 requests per burst, 10 per minute
			limiterMap[ip] = l
		}
		mu.Unlock()
		if !l.Allow() {
			return c.Error(429, errors.New("too many login attempts"))
		}
		return next(c)
	}
}

Register this middleware in actions/app.go:

func App() *buffalo.App {
	app := buffalo.New(buffalo.Options{
		Env:         ENV,
		SessionName: "_cms_session",
	})
	app.Use(middleware.SetLocale)
	app.Use(middleware.CSRF)
	app.Use(LoginRateLimiter) // Add before auth handlers
	app.GET("/", HomeHandler)
	auth := app.Group("/auth")
auth.Use(middleware.SetLocale)
auth.Use(middleware.CSRF)
auth.Get("/login", AuthController.Login)
auth.Post("/login", AuthController.LoginPost)
	return app
}

Second, prevent username enumeration by ensuring consistent error messages and timing. Modify the login handler to always perform a bcrypt comparison, even for non-existent users:

func (v AuthController) LoginPost(c buffalo.Context) error {
	u := &models.User{}
	if err := c.Bind(u); err != nil {
		return c.Error(400, err)
	}
	// Always query by email to avoid timing leaks
	foundUser := &models.User{}
	err := c.Value("tx").(*pop.Connection).Where("email = ?", u.Email).First(foundUser)
	// Compare hash regardless of whether user exists
	hash := "$2a$12$N9qo8uLOickgx2ZMRZoMyeGjZFyCI3RNRb/Q9F5JktJpGzJbG4F6W" // bcrypt of "password"
	if err != nil {
		// Simulate comparison to prevent timing leak
		bcrypt.CompareHashAndPassword([]byte(hash), []byte(u.Password))
		return c.Error(401, errors.New("invalid email or password"))
	}
	if err := bcrypt.CompareHashAndPassword([]byte(foundUser.PasswordHash), []byte(u.Password)); err != nil {
		return c.Error(401, errors.New("invalid email or password"))
	}
	// ... set session, redirect
	return nil
}

This approach ensures that whether the user exists or not, the application performs a bcrypt comparison and returns the same generic error, eliminating timing and response-based enumeration. Combined with IP-based rate limiting, this significantly raises the cost of credential stuffing attacks against Buffalo endpoints.