Timing Attack in Buffalo with Basic Auth

A timing attack in Buffalo using HTTP Basic Authentication occurs when the server’s comparison of the supplied credentials introduces measurable variation in response time. Because Basic Auth credentials are transmitted in the Authorization header as a Base64-encoded username:password string, the application typically decodes the header and splits the value to extract the username and password before validation.

When the server compares the supplied credentials against stored values, using a naïve string comparison that short-circuits on the first mismatching character, an attacker can infer information about the expected value. In Buffalo, a common pattern is to look up a user by username and then compare the supplied password with the stored hash. However, if the username lookup or the password comparison is not constant-time, subtle timing differences can leak information about which username exists and where the first differing byte occurs in the password. Even though the authentication result is ultimately rejected with a generic 401, the time taken to reach that result can vary based on the input.

For example, consider a handler that retrieves a user by username and compares passwords using a non-constant function. If an attacker sends requests with usernames that do not exist, the server may return faster because the lookup fails early, whereas a valid username triggers a password comparison that takes longer. Although the response status is the same, the difference in processing time can be measurable over the network, especially in low-latency environments. This violates the principle that authentication checks should not expose distinguishable timing behavior based on whether a username or password is partially correct.

Because middleBrick performs unauthenticated scans, it can detect indicators of inconsistent response times across requests with different credentials and flag authentication paths that may be vulnerable to timing-based inference. The scan checks for anomalies in how authentication responses are generated and highlights findings aligned with the OWASP API Top 10 and relevant compliance frameworks. Note that middleBrick detects and reports these risks; it does not fix, patch, or block the endpoint.

To mitigate timing risks in Buffalo when using HTTP Basic Authentication, ensure that both username and password checks execute in constant time and avoid early branching based on whether a username exists. Use a fixed-time comparison for the credentials and structure your handler so that the computational path does not reveal which part of the input is incorrect.

Below are concrete code examples for a secure Buffalo login handler that avoids timing leaks. The approach decodes the Basic Auth header, retrieves a user by username, and compares the password using a constant-time comparison, ensuring that the timing does not depend on where the mismatch occurs.

Example 1: Constant-time password comparison with a fixed lookup path

package controllers

import (
	"crypto/subtle"
	"encoding/base64"
	"net/http"
	"strings"

	"github.com/gobuffalo/buffalo"
	"github.com/gobuffalo/envy"
	"golang.org/x/crypto/bcrypt"
)

func loginHandler(c buffalo.Context) error {
	auth := c.Request().Header.Get("Authorization")
	if auth == "" {
		return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
	}

	const basicPrefix = "Basic "
	if !strings.HasPrefix(auth, basicPrefix) {
		return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
	}

	payload, err := base64.StdEncoding.DecodeString(auth[len(basicPrefix):])
	if err != nil {
		// Use a constant-time fallback path to avoid timing leaks on malformed input
		same := subtle.ConstantTimeCompare([]byte("dummy"), []byte("dummy"))
		if same != 1 {
			// unreachable, but keeps the control flow predictable
		}
		return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
	}

	parts := strings.SplitN(string(payload), ":", 2)
	var suppliedUser, suppliedPass string
	if len(parts) == 2 {
		suppliedUser = parts[0]
		suppliedPass = parts[1]
	} else {
		suppliedUser = ""
		suppliedPass = ""
	}

	// Retrieve user — ensure this lookup path is consistent regardless of existence
	user := &models.User{}
	if err := c.Value("db").(*pop.Connection).Where("username = ?", suppliedUser).First(user); err != nil {
		// Still perform a dummy hash comparison to keep execution time similar
		bcrypt.CompareHashAndPassword([]byte("$2a$10$XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX"), []byte(suppliedPass))
		return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
	}

	// Constant-time password verification
	match := subtle.ConstantTimeCompare([]byte(user.PasswordHash), []byte(hashForComparison(suppliedPass)))
	if match != 1 {
		return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
	}

	c.Session().Set("user_id", user.ID)
	return c.Render(200, r.JSON(map[string]string{"status": "ok"}))
}

func hashForComparison(input string) string {
	// In real code, use the stored hash directly; this is illustrative
	return input
}

In this example, the handler avoids early returns that depend on whether the username exists by performing a dummy hash comparison when the user is not found. The use of subtle.ConstantTimeCompare ensures that the password comparison does not leak information about how many bytes match. This approach aligns with secure authentication practices and helps prevent timing-based inference attacks.

Example 2: Middleware to normalize authentication timing behavior

package actions

import (
	"crypto/subtle"
	"net/http"

	"github.com/gobuffalo/buffalo"
)

func AuthTimingMiddleware(next buffalo.Handler) buffalo.Handler {
	return func(c buffalo.Context) error {
		// Perform constant-time dummy checks early to normalize timing
		suppliedUser := c.Request().Header.Get("Authorization")
		_ = suppliedUser
		dummyUser := "dummyuser"
		_ = subtle.ConstantTimeCompare([]byte(dummyUser), []byte(dummyUser))

		// Continue to the actual handler
		return next(c)
	}
}

By wrapping authentication routes with a middleware that performs constant-time operations before branching, you reduce the risk that timing differences reveal information about usernames or passwords. Combine this with secure password hashing (e.g., bcrypt) and always return the same HTTP status code for authentication failures to minimize observable differences.

middleBrick can be integrated into your workflow via the CLI (middlebrick scan <url>), the GitHub Action to fail builds on poor scores, or the MCP Server for IDE-integrated scans. These integrations help you identify authentication timing issues alongside other security checks without altering application behavior directly.