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simple-slack-verification

A simple express middleware for Slack signed secret authentication. See Slack's documentation on signing verification. This package also exposes a function for signature verification verifySignature for use outside of express.

If the request did not come from your Slack the middleware does not pass the request forward. On unauthorized requests it returns status 403 and the unauthorizedResponse object (see below).

Installation

npm i simple-slack-verification
yarn add simple-slack-verification

Usage

Since the protocol uses the body when generating the signature it is important that the body of the request is not modified at all before running this middleware.

// with no options
var express = require('express');
var slackVerification = require('simple-slack-verification');

// uses process.env.SLACK_SIGNING_SECRET
var app = express();
app.use( slackVerification() );
// with options
var express = require('express');
var slackVerification = require('simple-slack-verification');

var app = express();
app.use(
  slackVerification({
    secret: "YOUR SLACK SECRET, PLEASE STORE SAFELY",
    unauthorizedResponse: {
      code: 'unauthorized',
      message: 'Unauthorized request'
    },
    status: 200,
    maxSecondsOld: 60
  })
);

Options

slackVerification({ secret, unauthorizedResponse, status, maxSecondsOld })

Passed in as an object to the slackVerification function with the following fields. All of them are optional (including the object itself) and if omitted defaults to what's documented below:

  • secret: Your personal Slack secret. You can either give it in this parameter or just set it as the environment variable SLACK_SIGNING_SECRET. If this option is given the environment variable is not used.
    • Default: process.env.SLACK_SIGNING_SECRET.
    • Type: string
  • unauthorizedResponse: Sent in the response when unable to verify signature.
    • Default: { code: 'unauthorized', text: 'Unable to verify Slack request' }
    • Type: Anything res.send() allows.
  • status: The status code of the response when unable to verify signature.
    • Default: 403
    • Type: integer
  • maxSecondsOld: The max age of the message you allow, in seconds.
    • Default: 300, as per Slacks recommendation from their documentation.
    • Type: integer

Usage of verifySignature function

Note that this function does not verify the age of the timestamp like the middleware does.

var slackVerification = require('simple-slack-verification');
var verifySignature = slackVerification.verifySignature;

...

// uses process.env.SLACK_SIGNING_SECRET since 4th argument wasn't given
if (verifySignature(signature, timestamp, body)) {
  // valid request, do something with it
} else {
  // request did not come from your slack
}

Parameters

verifySignature(signature, timestamp, body, secret?)

Returns true if the signature is valid, false otherwise. First three arguments are required.

  • signature: The signature to verify, i.e from the x-slack-signature header.
    • Type: string
  • timestamp: The given timestamp of the request, i.e from the x-slack-request-timestamp header.
    • Type: string | integer
  • body: The body of the request, in express req.body.
    • Type: object
  • secret: Your personal Slack secret. This argument is optional and defaults to process.env.SLACK_SIGNING_SECRET.
    • Type: string

Security

Since this is a security-sensitive package here are some things done to increase security:

  • A maximum age on requests to avoid replay attacks. This is a recommended step from Slacks documentation.
  • Timing-safe comparisons of the signatures to avoid timing attacks. Also from Slacks documentation.
  • See more from Slack's documentation on the security of the protocol.

If you want please review the code yourself and open an issue if you think something is questionable.