Delivery webhooks with signatures

// Express: keep the raw body, check the signature, then answer 2xx quickly
import crypto from "node:crypto";
import express from "express";

const app = express();
const secret = process.env.BULKSMSAPI_WEBHOOK_SECRET; // whsec_...

app.post("/hooks/sms", express.raw({ type: "application/json" }), (req, res) => {
  const id = req.header("webhook-id");
  const ts = req.header("webhook-timestamp");
  const key = Buffer.from(secret.slice("whsec_".length), "base64url");
  const expected = crypto.createHmac("sha256", key).update(`${id}.${ts}.${req.body}`).digest("base64");
  const sent = (req.header("webhook-signature") || "").split(" ").map((s) => s.replace(/^v1,/, ""));
  const fresh = Math.abs(Date.now() / 1000 - Number(ts)) < 300;
  if (!fresh || !sent.some((s) => s.length === expected.length && crypto.timingSafeEqual(Buffer.from(s), Buffer.from(expected)))) {
    return res.status(401).end();
  }
  const event = JSON.parse(req.body);
  console.log(event.type, event.data.id, event.data.status);
  res.status(204).end();
});
app.listen(3000);

# Flask: check the signature on the raw body before parsing it
import base64, hashlib, hmac, json, os, time
from flask import Flask, abort, request

app = Flask(__name__)
SECRET = os.environ["BULKSMSAPI_WEBHOOK_SECRET"]  # whsec_...

@app.post("/hooks/sms")
def sms_hook():
    msg_id = request.headers.get("webhook-id", "")
    ts = request.headers.get("webhook-timestamp", "0")
    raw = request.get_data()
    key = base64.urlsafe_b64decode(SECRET[len("whsec_"):] + "==")
    signed = msg_id.encode() + b"." + ts.encode() + b"." + raw
    expected = base64.b64encode(hmac.new(key, signed, hashlib.sha256).digest()).decode()
    sent = [s.split(",", 1)[-1] for s in request.headers.get("webhook-signature", "").split()]
    if abs(time.time() - int(ts)) > 300 or not any(hmac.compare_digest(s, expected) for s in sent):
        abort(401)
    event = json.loads(raw)
    print(event["type"], event["data"]["id"], event["data"]["status"])
    return "", 204

<?php
// Plain PHP endpoint, for example /hooks/sms.php
$secret = getenv("BULKSMSAPI_WEBHOOK_SECRET"); // whsec_...
$raw = file_get_contents("php://input");
$id = $_SERVER["HTTP_WEBHOOK_ID"] ?? "";
$ts = $_SERVER["HTTP_WEBHOOK_TIMESTAMP"] ?? "0";
$key = base64_decode(strtr(substr($secret, 6), "-_", "+/"));
$expected = base64_encode(hash_hmac("sha256", "$id.$ts.$raw", $key, true));
$ok = false;
foreach (explode(" ", $_SERVER["HTTP_WEBHOOK_SIGNATURE"] ?? "") as $part) {
    $sig = substr($part, strpos($part, ",") + 1);
    if (hash_equals($expected, $sig)) $ok = true;
}
if (!$ok || abs(time() - (int)$ts) > 300) {
    http_response_code(401);
    exit;
}
$event = json_decode($raw, true);
error_log($event["type"] . " " . $event["data"]["id"]);
http_response_code(204);

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"encoding/json"
	"io"
	"log"
	"math"
	"net/http"
	"os"
	"strconv"
	"strings"
	"time"
)

func smsHook(w http.ResponseWriter, r *http.Request) {
	secret := os.Getenv("BULKSMSAPI_WEBHOOK_SECRET") // whsec_...
	key, _ := base64.RawURLEncoding.DecodeString(strings.TrimPrefix(secret, "whsec_"))
	raw, _ := io.ReadAll(r.Body)
	id, ts := r.Header.Get("webhook-id"), r.Header.Get("webhook-timestamp")
	mac := hmac.New(sha256.New, key)
	mac.Write([]byte(id + "." + ts + "." + string(raw)))
	expected := base64.StdEncoding.EncodeToString(mac.Sum(nil))
	sec, _ := strconv.ParseInt(ts, 10, 64)
	ok := false
	for _, part := range strings.Fields(r.Header.Get("webhook-signature")) {
		sig := part[strings.Index(part, ",")+1:]
		if hmac.Equal([]byte(sig), []byte(expected)) {
			ok = true
		}
	}
	if !ok || math.Abs(float64(time.Now().Unix()-sec)) > 300 {
		w.WriteHeader(http.StatusUnauthorized)
		return
	}
	var event struct {
		Type string
		Data map[string]any
	}
	json.Unmarshal(raw, &event)
	log.Println(event.Type, event.Data["id"], event.Data["status"])
	w.WriteHeader(http.StatusNoContent)
}

func main() {
	http.HandleFunc("/hooks/sms", smsHook)
	log.Fatal(http.ListenAndServe(":3000", nil))
}