Rate Limiting Bypass in Echo Go with Hmac Signatures

Rate limiting is a common control to protect APIs from abuse. When an API uses HMAC signatures for request authentication in Echo Go, implementation decisions can unintentionally weaken rate limiting. A typical vulnerability occurs when the signature is computed over a subset of request properties—such as excluding the client identifier or a nonce—so that distinct requests from the same client can produce identical signatures. If the server enforces limits per signature value rather than per authenticated identity, an attacker can rotate request parameters (e.g., timestamps or nonces) that are excluded from the HMAC, causing each request to appear as a new, valid signature to the server. This allows the attacker to bypass rate limits because the server counts each unique signature independently.

Another specific risk arises when the HMAC includes only the request body or selected headers but omits critical dimensions such as the HTTP method, path, or an API key identifier that is not part of the signature. In Echo Go, if middleware validates the signature and then applies rate limiting based on the raw client IP or an insufficient key, an attacker can send many requests with different IPs or distributed sources while keeping the signed components constant. The server may also enforce rate limits before signature validation, allowing an unauthenticated flood of requests to consume server-side resources before the HMAC check occurs. This can lead to denial of service or enable brute-force attempts against other protections.

Consider a service that signs requests using HMAC-SHA256 over a canonical string composed of HTTP method, path, and a timestamp, but excludes a client ID or an API key. An attacker can reuse the same signed payload within a window while varying the client IP or using multiple endpoints. If Echo Go’s rate limiter is configured per route without tying it to the authenticated identity derived from the HMAC, the limit can be circumvented. Real-world patterns seen in the wild include missing nonce or replay protection, which allows captured requests to be replayed within the rate limit window. The result is a bypass where the attacker appears to comply with per-signature rate limits while exhausting the server-side quota.

To remediate rate limiting bypass risks in Echo Go when using HMAC signatures, you must tie rate limiting to the authenticated identity and ensure the signed scope covers all dimensions that define a unique request. Avoid computing the HMAC over a partial set of attributes. Include the client identifier, HTTP method, path, timestamp, and a nonce or request-specific unique value in the signature base string. This ensures that each distinct request yields a different signature, preventing signature reuse across requests that should be counted separately.

Below is a concrete example of signing and verification in Echo Go that incorporates these principles. The code shows how to build a canonical string, compute HMAC-SHA256, and validate both signature and replay/rate-limiting constraints within middleware.

// HMAC signing and verification in Echo Go (secure pattern)
package main

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

	"github.com/labstack/echo/v4"
)

const (
	// Use a per-client or per-key secret stored securely
	hmacSecret = "example-secret-key"
	// Allow a small clock skew
	maxTimestampSkew = 5 * 60 
)

// buildCanonicalString creates a deterministic string for HMAC
func buildCanonicalString(method, path, timestamp, nonce, clientID string, body string) string {
	// Include all dimensions that define request uniqueness
	parts := []string{method, path, clientID, timestamp, nonce, body}
	return strings.Join(parts, "|")
}

// verifyHMAC validates the signature and enforces replay/rate-limiting hints
func verifyHMAC(next echo.HandlerFunc) echo.HandlerFunc {
	return func(c echo.Context) error {
		clientID := c.Request().Header.Get("X-Client-ID")
		timestamp := c.Request().Header.Get("X-Timestamp")
		nonce := c.Request().Header.Get("X-Nonce")
		signature := c.Request().Header.Get("X-Signature")

		if clientID == "" || timestamp == "" || nonce == "" || signature == "" {
			return echo.NewHTTPError(http.StatusBadRequest, "missing authentication headers")
		}

		// Reject old timestamps to prevent replay
		ts, err := strconv.ParseInt(timestamp, 10, 64)
		if err != nil || time.Now().Unix()-ts > maxTimestampSkew {
			return echo.NewHTTPError(http.StatusForbidden, "timestamp skew or replay detected")
		}

		// Read body safely if needed for signing (be mindful of echo.Body limitation)
		bodyBytes, _ := c.Get("__raw_body").([]byte)
		bodyStr := string(bodyBytes)

		canonical := buildCanonicalString(c.Request().Method, c.Request().RequestURI, timestamp, nonce, clientID, bodyStr)
		mac := hmac.New(sha256.New, []byte(hmacSecret))
		mac.Write([]byte(canonical))
		expected := hex.EncodeToString(mac.Sum(nil))

		if !hmac.Equal([]byte(expected), []byte(signature)) {
			return echo.NewHTTPError(http.StatusUnauthorized, "invalid signature")
		}

		// At this point, the request is authenticated.
		// Attach clientID to context so rate limiting can be enforced per identity.
		c.Set("clientID", clientID)
		return next(c)
	}
}

In this pattern, the canonical string includes clientID, method, path, timestamp, nonce, and body. The server must enforce rate limits based on clientID (or API key) rather than on the signature value or IP alone. Additionally, maintain a short cache of recently used nonces per clientID with a TTL slightly larger than the allowed request window to prevent replay attacks that would circumvent per-request uniqueness. This ensures that even if timestamps are reused accidentally, the nonce guarantees a distinct canonical input and signature.

When integrating with Echo Go’s middleware chain, place verifyHMAC before your rate-limiting middleware so that the authenticated identity is available for accurate counting. Avoid computing the HMAC over only the body or only selected headers, as that enables signature reuse across different methods or paths. By binding rate limiting to the authenticated identity derived from a fully scoped HMAC, you reduce the risk of bypass and ensure that limits reflect the true client behavior.