Webhook Abuse in Echo Go with Hmac Signatures

Webhook abuse in an Echo Go service using HMAC signatures can occur when signature validation is incomplete or inconsistently applied. Echo is a popular HTTP router for Go, and webhooks are commonly implemented by verifying an HMAC-SHA256 signature provided in a request header to ensure the payload originates from a trusted sender.

In a vulnerable implementation, the server may compute the HMAC over only a subset of the data, such as the raw body bytes, but then compare the provided signature using a non-constant-time function. This opens the door to timing attacks where an attacker can iteratively guess the signature byte-by-byte. Additionally, if the server fails to validate the signature before processing business logic, it may process duplicated or replayed webhook events, leading to duplicate resource creation or unintended state changes.

Another common pitfall is poor key management: storing the shared secret in environment variables that are accidentally logged or exposed through debug endpoints increases the risk of signature forgery. If the Echo route handling the webhook does not enforce strict content-type checks, an attacker may send malformed content types that bypass normalization steps, causing the server to compute a different HMAC than expected and potentially disabling validation altogether in some configurations.

Consider a typical vulnerable handler:

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"io"
	"net/http"
	"os"

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

func unsafeWebhook(c echo.Context) error {
	secret := []byte(os.Getenv("WEBHOOK_SECRET"))
	body, err := io.ReadAll(c.Request().Body)
	if err != nil {
		return err
	}
	// Vulnerable: no constant-time compare, no replay protection
	mac := hmac.New(sha256.New, secret)
	mac.Write(body)
	expected := mac.Sum(nil)
	provided := []byte(c.Request().Header.Get("X-Signature"))
	if !hmac.Equal(expected, provided) {
		return c.String(http.StatusUnauthorized, "invalid signature")
	}
	// Vulnerable: processing after late validation, no idempotency
	var payload map[string]interface{}
	if err := json.Unmarshal(body, &payload); err != nil {
		return err
	}
	// business logic that may execute multiple times for same event
	return c.JSON(http.StatusOK, map[string]string{"status": "received"})
}

An attacker who can observe responses for timing differences can recover the HMAC, and once recovered, can forge webhook events that the server will accept. Replayed requests can cause duplicate payments or state mutations if idempotency is not enforced at the application level.