Man In The Middle in Buffalo with Hmac Signatures

A Man In The Middle (MITM) attack against a Buffalo application that uses Hmac Signatures can occur when an attacker intercepts communication between a client and the server and tampers with in-flight requests. Buffalo applications often use Hmac Signatures to verify the integrity and origin of requests, commonly by signing a subset of request data (e.g., parameters, timestamp, or a canonical string) with a shared secret and including the signature in headers. If the application does not enforce strict validation and does not protect the transmission path, an attacker who can observe and modify traffic may alter the request body or parameters and attempt to forge a valid Hmac Signature.

Consider a Buffalo API endpoint that relies on an Hmac-SHA256 signature passed via a custom header. The client computes signature = HmacSha256(secret, canonical_string), where canonical_string might include selected headers, the request path, and a timestamp. The server recomputes the signature using the same method and compares it with the provided header using a constant-time compare. A MITM can exploit weaknesses in this flow in several ways:

• If the server accepts requests without verifying the presence and correctness of the Hmac Signature, an attacker can modify parameters (such as user ID or permissions) and the server may process the tampered request as valid.
• If the server uses a weak or predictable canonicalization method (e.g., including only some headers or failing to enforce HTTPS), the attacker may reconstruct or guess a valid signature for modified content.
• If the server does not enforce HTTPS (transport layer protection), the secret used for Hmac Signatures is not protected in transit, and an attacker can more easily mount offline guessing or replay attacks.
• Replay attacks are also possible if the server does not validate timestamps or nonces alongside the Hmac Signature, allowing a captured signed request to be re-sent maliciously.

In Buffalo, these issues are not inherent to Hmac Signatures, but arise from implementation choices: missing transport security, incomplete signature coverage, missing replay protection, or incorrect comparison logic. Because Hmac Signatures rely on a shared secret, exposing any part of the protocol that allows an attacker to observe or influence the signed data can compromise integrity. Using middleBrick, you can scan such endpoints to detect missing integrity checks, weak canonicalization, and missing transport protections as part of the Input Validation and Authentication checks.

To remediate MITM risks when using Hmac Signatures in Buffalo, enforce HTTPS, use a strong canonicalization strategy, include a timestamp or nonce, and perform constant-time comparison. Below are concrete Go examples aligned with Buffalo’s patterns.

1. Enforce HTTPS and secure transport

Ensure your Buffalo application redirects HTTP to HTTPS and sets secure headers. In actions/app.go, you can add a before filter to require TLS in production:

// actions/app.go
func app() *buffalo.App {
  app := buffalo.New(buffalo.Options{
    Env:         ENV,
    SessionStore: &sessions.CookieStore{},
  })

  // Enforce HTTPS in production to prevent MITM
  if app.Env() == environments.Production {
    app.Use(forceSSL)
  }

  app.Before(plugins.EnsureSession)
  // ... register routes and middleware
  return app
}

func forceSSL(next buffalo.Handler) buffalo.Handler {
  return func(c buffalo.Context) error {
    if c.Request().TLS == nil {
      c.Response().WriteHeader(http.StatusMovedPermanently)
      c.Response().Header().Set("Location", "https://"+c.Request().Host+c.Request().RequestURI)
      return nil
    }
    return next(c)
  }
}

2. Canonicalization and signing with Hmac-SHA256

Define a canonical string that includes the request path, selected headers, and a timestamp. Sign it with a shared secret and include the signature and timestamp in headers. Here is an example client-side signing helper and server-side verification within a Buffalo before action.

// client_side.go
package main

import (
  "crypto/hmac"
  "crypto/sha256"
  "encoding/hex"
  "net/http"
  "strings"
  "time"
)

func signRequest(req *http.Request, secret string) {
  timestamp := strconv.FormatInt(time.Now().Unix(), 10)
  // canonical: METHOD + path + timestamp
  canonical := strings.ToUpper(req.Method) + "
" + req.URL.Path + "
" + timestamp
  mac := hmac.New(sha256.New, []byte(secret))
  mac.Write([]byte(canonical))
  signature := hex.EncodeToString(mac.Sum(nil))
  req.Header.Set("X-Request-Timestamp", timestamp)
  req.Header.Set("X-Request-Signature", signature)
}

// server_side.go — inside a Buffalo before action
func verifyHmac(next buffalo.Handler) buffalo.Handler {
  return func(c buffalo.Context) error {
    const secret = "super-secret-shared" // use secure env/config
    ts := c.Request().Header.Get("X-Request-Timestamp")
    sig := c.Request().Header.Get("X-Request-Signature")
    if ts == "" || sig == "" {
      c.Response().WriteHeader(http.StatusUnauthorized)
      return errors.New("missing signature or timestamp") // use proper error handling
    }
    // Recompute canonical
    canonical := strings.ToUpper(c.Request().Method) + "
" + c.Request().URL.Path + "
" + ts
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(canonical))
    expected := hex.EncodeToString(mac.Sum(nil))
    // Use subtle.ConstantTimeCompare to avoid timing attacks
    if !hmac.Equal([]byte(expected), []byte(sig)) {
      c.Response().WriteHeader(http.StatusForbidden)
      return errors.New("invalid signature") // use proper error handling
    }
    // Optional: reject stale timestamps (e.g., >2 minutes) to prevent replay
    // t, _ := time.Parse("Unix", ts)
    // if time.Since(t) > 2*time.Minute { ... reject ... }
    return next(c)
  }
}

3. Include nonce or replay protection

To prevent replay, include a nonce or short timestamp window and track recently seen nonces. This complements Hmac Signatures by ensuring freshness:

// server_side.go — extended verification with nonce/replay cache
var seenNonces = struct {
  sync.Mutex
  m map[string]bool
}{m: make(map[string]bool)}

func verifyHmacWithNonce(next buffalo.Handler) buffalo.Handler {
  return func(c buffalo.Context) error {
    ts := c.Request().Header.Get("X-Request-Timestamp")
    sig := c.Request().Header.Get("X-Request-Signature")
    nonce := c.Request().Header.Get("X-Request-Nonce")
    if ts == "" || sig == "" || nonce == "" {
      c.Response().WriteHeader(http.StatusUnauthorized)
      return errors.New("missing signature, timestamp, or nonce") // use proper error handling
    }
    // Reject replayed nonces within a window
    seenNonces.Lock()
    if seenNonces.m[nonce] {
      seenNonces.Unlock()
      c.Response().WriteHeader(http.StatusForbidden)
      return errors.New("replay detected") // use proper error handling
    }
    seenNonces.m[nonce] = true
    seenNonces.Unlock()
    // Clean old nonces periodically in production
    canonical := strings.ToUpper(c.Request().Method) + "
" + c.Request().URL.Path + "
" + ts + "
" + nonce
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(canonical))
    expected := hex.EncodeToString(mac.Sum(nil))
    if !hmac.Equal([]byte(expected), []byte(sig)) {
      c.Response().WriteHeader(http.StatusForbidden)
      return errors.New("invalid signature") // use proper error handling
    }
    return next(c)
  }
}

These steps ensure that Hmac Signatures in Buffalo are used with transport security, robust canonicalization, freshness, and constant-time verification, reducing the risk of successful MITM tampering. middleBrick’s scans can validate these controls under Authentication and Input Validation checks.