express-basic-auth

Simple plug & play HTTP basic auth middleware for Express.

How to install

Just run

npm install @blossomfinance/express-basic-auth

How to use

The module will export a function, that you can call with an options object to get the middleware:

const app = require('express')()
const basicAuth = require('@blossomfinance/express-basic-auth')

app.use(basicAuth({
    users: { 'admin': 'supersecret' }
}))

The middleware will now check incoming requests to match the credentials admin:supersecret.

The middleware will check incoming requests for a basic auth (Authorization) header, parse it and check if the credentials are legit. If there are any credentials, an auth property will be added to the request, containing an object with user and password properties, filled with the credentials, no matter if they are legit or not.

If a request is found to not be authorized, it will respond with HTTP 401 and a configurable body (default empty).

Static Users

If you simply want to check basic auth against one or multiple static credentials, you can pass those credentials in the users option:

app.use(basicAuth({
    users: {
        'admin': 'supersecret',
        'adam': 'password1234',
        'eve': 'asdfghjkl',
    }
}))

The middleware will check incoming requests to have a basic auth header matching one of the three passed credentials.

Custom authorization

Alternatively, you can pass your own authorizer function, to check the credentials however you want. It will be called with a username and password and is expected to return true or false to indicate that the credentials were approved or not.

When using your own authorizer, make sure not to use standard string comparison (== / ===) when comparing user input with secret credentials, as that would make you vulnerable against timing attacks. Use the provided safeCompare function instead - always provide the user input as its first argument. Also make sure to use bitwise logic operators (| and &) instead of the standard ones (|| and &&) for the same reason, as the standard ones use shortcuts.

app.use(basicAuth( { authorizer: myAuthorizer } ))

function myAuthorizer(username, password) {
    const userMatches = basicAuth.safeCompare(username, 'customuser')
    const passwordMatches = basicAuth.safeCompare(password, 'custompassword')

    return userMatches & passwordMatches
}

This will authorize all requests with the credentials 'customuser:custompassword'. In an actual application you would likely look up some data instead ;-) You can do whatever you want in custom authorizers, just return true or false in the end and stay aware of timing attacks.

Custom Async Authorization

Note that the authorizer function above is expected to be synchronous. This is the default behavior, you can pass authorizeAsync: true in the options object to indicate that your authorizer is asynchronous. In this case it will be passed a callback as the third parameter, which is expected to be called by standard node convention with an error and a boolean to indicate if the credentials have been approved or not. Let's look at the same authorizer again, but this time asynchronous:

app.use(basicAuth({
    authorizer: myAsyncAuthorizer,
    authorizeAsync: true,
}))

function myAsyncAuthorizer(username, password, cb) {
    if (username.startsWith('A') & password.startsWith('secret'))
        return cb(null, true)
    else
        return cb(null, false)
}

Unauthorized Response Body

Per default, the response body for unauthorized responses will be empty. It can be configured using the unauthorizedResponse option.

You can pass one of the following:

• a static value (string or object)
• a function that synchronously returns a value (string or object)
• a middleware

If a string value is returned or produced by the function, it will be sent as-is as the response along with a 401 status code. Ohterwise, it will be sent as json via the .json method.

// static value
app.use(basicAuth({
    users: { 'Foo': 'bar' },
    unauthorizedResponse: 'Sorry, you are not authorized.'
}))

// a function that synchronously returns a value
app.use(basicAuth({
    users: { 'Foo': 'bar' },
    unauthorizedResponse: function (req) {
        return req.auth
            ? ('Credentials ' + req.auth.user + ':' + req.auth.password + ' rejected')
            : 'No credentials provided'
    }
}))

// a middleware
app.use(basicAuth({
    users: { 'Foo': 'bar' },
    unauthorizedResponse: function (req, res, next) {
      if (req.auth) {
          next(new Error('Login required'));
          return;
      }
      // or send an html page
      if (req.accepts('html') {
          res.render('unauthorized');
          return;
      }
      // or whatever you like!
      res.status(403)
          .set('X-Toynbee-Idea', 'In Kubrick’s 2001 Resurrect Dead On Planet Jupiter')
          .send({
            message: 'Future authorization attempts are futile.'
          });
    }
}))

Challenge

Per default the middleware will not add a WWW-Authenticate challenge header to responses of unauthorized requests. You can enable that by adding challenge: true to the options object. This will cause most browsers to show a popup to enter credentials on unauthorized responses. You can set the realm (the realm identifies the system to authenticate against and can be used by clients to save credentials) of the challenge by passing a static string or a function that gets passed the request object and is expected to return the challenge:

app.use(basicAuth({
    users: { 'someuser': 'somepassword' },
    challenge: true,
    realm: 'Imb4T3st4pp',
}))

Try it

The repository contains an example.js that you can run to play around and try the middleware. To use it just put it somewhere (or leave it where it is), run

npm install express @blossomfinance/express-basic-auth
node example.js

This will start a small express server listening at port 8080. Just look at the file, try out the requests and play around with the options.

TypeScript usage

A declaration file is bundled with the library. You don't have to install a @types/ package.

import * as basicAuth from 'express-basic-auth'

:bulb: Using req.auth

express-basic-auth sets req.auth to an object containing the authorized credentials like { user: 'admin', password: 'supersecret' }.

In order to use that req.auth property in TypeScript without an unknown property error, use covariance to downcast the request type:

app.use(basicAuth(options), (req: basicAuth.IBasicAuthedRequest, res, next) => {
    res.end(`Welcome ${req.auth.user} (your password is ${req.auth.password})`)
    next()
})

:bulb: A note about type inference on synchronous authorizers

Due to some TypeScript's type-system limitation, the arguments' type of the synchronous authorizers are not inferred. For example, on an asynchronous authorizer, the three arguments are correctly inferred:

basicAuth({
    authorizeAsync: true,
    authorizer: (user, password, authorize) => authorize(null, password == 'secret'),
})

However, on a synchronous authorizer, you'll have to type the arguments yourself:

basicAuth({
    authorizer: (user: string, password: string) => (password == 'secret')
})

Tests

The cases in the example.js are also used for automated testing. So if you want
to contribute or just make sure that the package still works, simply run:

npm test