Microsoft Teams Outgoing Webhook Proxy for Microsoft Flow

flow-bot-azure-function.js

const crypto = require('crypto');
const request = require('request');

const bufSecret = Buffer('OUTGOING-WEBHOOK-SECRET', "base64");
const flowWebhook = 'MICROSOFT-FLOW-URL';

module.exports = function (context, req) {
    var auth = req.headers['authorization'];

    // HMAC security validation
    var msgBuf = Buffer.from(req.rawBody, 'utf8');
    var msgHash = "HMAC " + crypto.createHmac('sha256', bufSecret).update(msgBuf).digest("base64");

    if (msgHash == auth) {
        request.post(flowWebhook, {
            body: req.rawBody,
            headers: { 'content-type': 'application/json' },
        }, function (error, response, body) {
            context.res = {
                headers: { 'content-type': 'application/json' },
                body: JSON.stringify({
                    "type": "message",
                    "text": "Thank you!"
                })
            };
            context.done();
        });

    } else {
        context.res = {
            status: 200,
            headers: { 'content-type': 'application/json' },
            body: JSON.stringify({
                "type": "message",
                "text": "Error: message sender cannot be authenticated."
            })
        }
        context.done();
    }

};