Rate Limiting Bypass in Buffalo with Hmac Signatures

Buffalo is a Go web framework commonly used to build APIs that use HMAC signatures for request authentication. When HMAC is used to sign requests, the typical design includes a timestamp, a nonce, and selected headers (often the request path and body) combined with a shared secret to produce a signature that is passed in a header (e.g., X-API-Signature). If rate limiting is applied only after signature verification and does not account for signature uniqueness or replay potential, an attacker can bypass intended rate limits by varying elements that change the signature without changing the effective identity of the caller.

A concrete bypass scenario: an API endpoint protected by HMAC signatures may also enforce rate limits based on a static API key or IP address. However, if the HMAC signature includes a timestamp with second-level granularity and the server enforces per-minute limits per signature, an attacker can generate a new signature for each second by changing the timestamp and possibly a nonce. Because each signed request appears unique to the rate limiter, the attacker can issue many distinct signed requests that each stay under the per-minute threshold, effectively circumventing the intended limit. This becomes more likely when the server does not enforce replay protection (e.g., rejecting recent nonces) and does not scope rate limits to the identity derived from the HMAC key rather than the mutable request attributes.

Additionally, if the signature scope is too narrow (for example, only the HTTP method and path, excluding headers or body), an attacker can vary non-signature dimensions that the rate limiter counts independently. For instance, including an X-Request-ID that is not part of the HMAC will change request identity for the rate limiter while the signature remains valid, enabling rapid sequential requests that appear legitimate. Because Buffalo does not prescribe how to scope rate limiting with respect to authenticated requests, developers may inadvertently couple authentication correctness with rate limiting in a way that allows identity-spoofing or replay-based bypass. Real-world patterns seen in API abuse include timestamp manipulation, nonce reuse, and header manipulation to inflate request volume without triggering alerts.

To mitigate rate limiting bypass when using HMAC signatures in Buffalo, scope rate limiting to the authenticated principal (the entity that owns the HMAC key) rather than mutable request properties. Ensure the rate limiter operates before or in parallel with signature validation and uses a stable identifier extracted from the authenticated context (e.g., API key ID or a canonical user ID derived from the HMAC payload). Do not rely on timestamps or nonces alone to enforce limits; instead, use a deterministic identifier that does not change across legitimately signed requests.

Below are concrete code examples demonstrating HMAC signing in Buffalo and how to integrate rate limiting with a stable principal. The first example shows how to compute and verify an HMAC signature using SHA256 in Go, and the second shows how to structure middleware that extracts a principal and enforces rate limits before routing.

Example: HMAC signing and verification in Buffalo (Go)

package main

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

// computeSignature returns a hex-encoded HMAC-SHA256 signature
// canonicalString should be built from selected parts of the request,
// e.g., timestamp + nonce + HTTP method + path + body.
func computeSignature(secret, canonicalString string) string {
	h := hmac.New(sha256.New, []byte(secret))
	h.Write([]byte(canonicalString))
	return hex.EncodeToString(h.Sum(nil))
}

// buildCanonicalString demonstrates a deterministic construction.
// Include elements required for replay protection and stable identification.
func buildCanonicalString(timestamp string, nonce, method, path, body string) string {
	// Important: sort query parameters if present and include them canonically.
	// Keep scope narrow but consistent between client and server.
	return fmt.Sprintf("%s|%s|%s|%s|%s", timestamp, nonce, method, path, body)
}

// verifySignature validates the request signature and returns the parsed timestamp+nonce.
func verifySignature(secret, canonical, receivedSig string) (string, string, bool) {
	expected := computeSignature(secret, canonical)
	if !hmac.Equal([]byte(expected), []byte(receivedSig)) {
		return "", "", false
	}
	// Parse canonical to extract timestamp and nonce for replay checks.
	parts := strings.Split(canonical, "|")
	if len(parts) < 2 {
		return "", "", false
	}
	return parts[0], parts[1], true
}

Buffalo middleware integrating HMAC verification and stable-rate limiting

package main

import (
	"net/http"
	"strings"
	"time"

	"github.com/gobuffalo/buffalo"
	"github.com/gobuffalo/buffalo/middleware"
	"github.com/gobuffalo/packr/v2"
	"github.com/redis/go-redis/v9"
)

// principalFromRequest extracts a stable identifier (e.g., API key ID)
// from an authenticated context. In practice this would come from a lookup
// keyed by the HMAC key ID included in the signed headers.
func principalFromRequest(r *http.Request) string {
	h := r.Header.Get("X-API-Key")
	if h == "" {
		// fallback to a stable value derived after HMAC verification
		return "unknown"
	}
	return h
}

// RateLimitWithHMAC returns a buffalo middleware that enforces rate limits
// scoped to the authenticated principal, not mutable request attributes.
func RateLimitWithHMAC(next buffalo.Handler) buffalo.Handler {
	client := redis.NewClient(&redis.Options{
		Addr: "localhost:6379",
	})
	return func(c buffalo.Context) error {
		// Perform HMAC verification first; if invalid, reject early.
		// (Assume helper validateHMAC populates identity.)
		ok := validateHMAC(c.Request())
		if !ok {
			c.Response().WriteHeader(http.StatusUnauthorized)
			return nil
		}
		principal := principalFromRequest(c.Request())
		if principal == "unknown" {
			c.Response().WriteHeader(http.StatusBadRequest)
			return nil
		}
		// Use a stable key tied to principal, e.g., "rate:hmac::minute"
		key := "rate:hmac:" + principal + ":" + time.Now().Truncate(time.Minute).Format("200601021504")
		cur, err := client.Incr(c.Request().Context(), key).Result()
		if err != nil {
			c.Response().WriteHeader(http.StatusInternalServerError)
			return nil
		}
		if cur == 1 {
			// First request in this window; set expiry to avoid stale keys.
			client.Expire(c.Request().Context(), key, 2*time.Minute)
		}
		const limit = 60
		if cur > limit {
			c.Response().WriteHeader(http.StatusTooManyRequests)
			return nil
		}
		return next(c)
	}
}

// validateHMAC is a placeholder for full HMAC verification using the scheme above.
func validateHMAC(r *http.Request) bool {
	// Extract timestamp, nonce, signature headers, reconstruct canonical,
	// verify with shared secret, and optionally check replay cache.
	return true
}

In these examples, rate limiting is applied per principal derived from the authenticated HMAC context rather than per IP or mutable headers. This prevents attackers from changing timestamps or nonces to generate new signatures that would circumvent counts. Additionally, enforce replay protection by storing used nonces or timestamps for a window and rejecting duplicates. Combine this with server-side canonicalization that excludes non-signature-modifiable fields from the signed scope to reduce confusion in what the rate limiter counts as a unique request.