Verify GitHub webhook signature header in Node.js

githook.js

/*
 * Verify GitHub webhook signature header in Node.js
 * Written by stigok and others (see gist link for contributor comments)
 * https://gist.github.com/stigok/57d075c1cf2a609cb758898c0b202428
 * Licensed CC0 1.0 Universal
 */

const crypto = require('crypto')
const express = require('express')
const bodyParser = require('body-parser')

const secret = CHANGE_ME;

// For these headers, a sigHashAlg of sha1 must be used instead of sha256
// GitHub: X-Hub-Signature
// Gogs:   X-Gogs-Signature
const sigHeaderName = 'X-Hub-Signature-256'
const sigHashAlg = 'sha256'

const app = express()

// Saves a valid raw JSON body to req.rawBody
// Credits to https://stackoverflow.com/a/35651853/90674
app.use(bodyParser.json({
  verify: (req, res, buf, encoding) => {
    if (buf && buf.length) {
      req.rawBody = buf.toString(encoding || 'utf8');
    }
  },
}))

function verifyPostData(req, res, next) {
  if (!req.rawBody) {
    return next('Request body empty')
  }

  const sig = Buffer.from(req.get(sigHeaderName) || '', 'utf8')
  const hmac = crypto.createHmac(sigHashAlg, secret)
  const digest = Buffer.from(sigHashAlg + '=' + hmac.update(req.rawBody).digest('hex'), 'utf8')
  if (sig.length !== digest.length || !crypto.timingSafeEqual(digest, sig)) {
    return next(`Request body digest (${digest}) did not match ${sigHeaderName} (${sig})`)
  }

  return next()
}

app.post('/', verifyPostData, function (req, res) {
  res.status(200).send('Request body was signed')
})

app.use((err, req, res, next) => {
  if (err) console.error(err)
  res.status(403).send('Request body was not signed or verification failed')
})

app.listen(3000, () => console.log("Listening on port 3000"))

I am unable to reproduce this on Linux with Node.js v10.16.1 and express@4.17.1. Maybe this is some Windows-specific issue. Good luck :)

@stigok - I'd say you are right. Thanks for checking!

Thanks.

How "Length Extension Attack" is mitigated in this code?

Thank you for this! Worked perfectly.

@roaste

How "Length Extension Attack" is mitigated in this code?

This solution is using HMAC, which is not susceptible to length extension attacks.

Note that since HMAC doesn't use this construction, HMAC hashes are not prone to length extension attacks.[5]
Wikipedia

Thank for the help.
For me it only worked when using req.body instead of JSON.stringify(req.body) when calling hmac.update(payload)

@pimverkerk that sounds strange. When using bodyParser.json() it should only be parsing requests with Content-Type: application/json, and req.body should be a JavaScript object. Are you using GitHub webhooks, and otherwise using a verbatim copy of this code?

Hi @stigok, thanks for sharing this!

I used this code for a different purpose, not for Github webhooks. In my case, I found that stringifying the parsed body does not necessarily return the exact original request body. Consider for example JSON.stringify(JSON.parse("{\"value\":1.0}")), which returns {"value":1}. I don't know if this happens with the particular data sent by the Github webhooks, but if that is the case, I found the solutions here to be helpful https://stackoverflow.com/a/35651853/1531992

@ludcila oh, I see. So this can become a problem when verifying the signature. In JavaScript (1 === 1.0) === true, and 1.0 will be represented as 1 between de- and serialisation. Thanks for reporting. I'll see what I can do here.

works with sha256 too.

I would suggest changing it to sha256 and using the header X-Hub-Signature-256 as it's the recommendation from GitHub

@andrefcdias could you please post a reference?

@stigok sure! Here's the docs

I would suggest changing it to sha256 and using the header X-Hub-Signature-256 as it's the recommendation from GitHub

@andrefcdias thank you! I've updated the code with new header name and hash alg.

Hi @stigok, thanks for sharing this!

I used this code for a different purpose, not for Github webhooks. In my case, I found that stringifying the parsed body does not necessarily return the exact original request body. Consider for example JSON.stringify(JSON.parse("{\"value\":1.0}")), which returns {"value":1}. I don't know if this happens with the particular data sent by the Github webhooks, but if that is the case, I found the solutions here to be helpful https://stackoverflow.com/a/35651853/1531992

Thanks again! Now imlemented!

thank you :)

Thank you for sharing this, @stigok!

Copy-paste for Typescript with express 4.16+

import express, { NextFunction, Request, Response } from 'express';
import crypto from 'crypto';

const secret = process.env.SECRET;
const sigHeaderName = 'X-Hub-Signature-256';
const sigHashAlg = 'sha256';

function verifyPayload(req: Request, res: Response, next: NextFunction): void {
  if (!req.body) {
    return next('Request body empty');
  }
  const data = JSON.stringify(req.body);
  const sig = Buffer.from(req.get(sigHeaderName) || '', 'utf8');
  const hmac = crypto.createHmac(sigHashAlg, secret);
  const digest = Buffer.from(`${sigHashAlg}=${hmac.update(data).digest('hex')}`, 'utf8');
  if (sig.length !== digest.length || !crypto.timingSafeEqual(digest, sig)) {
    return next(`Request body digest (${digest}) did not match ${sigHeaderName} (${sig})`);
  }
  return next();
}

export const app = express();

app.use(express.json());

app.post('/', verifyPayload, (req: Request, res: Response) => {
  res.status(200).send('Request body was signed');
});

app.use((err: any, req: Request, res: Response, next: NextFunction) => {
  if (err) {
    console.error(err);
  }
  res.status(403).send('Request body was not signed or verification failed');
});

Thanks a lot !!!!

Does the request body get sent as a base64 encoded string? I'm trying to write an AWS lambda function to deal with this. Not sure if this is something that happens with AWS in general or if the payload actually gets sent as a base64 encoded string.

Does the request body get sent as a base64 encoded string? I'm trying to write an AWS lambda function to deal with this. Not sure if this is something that happens with AWS in general or if the payload actually gets sent as a base64 encoded string.

I recommend you read the API documentation for AWS lambda to find an answer to this. This code works as-is for incoming HTTP GitHub webhook requests.

I'm trying to figure this out while using Cloudflare Workers (and itty-router instead of express), anyone have any suggestions?