basic_auth_overwrite.rb

require 'base64'
require 'uri'
require 'faraday'

module FaradayMiddleware
  class BasicAuthOverwrite < Faraday::Middleware
    KEY = "Authorization".freeze

    def call(env)
      rewrite_request(env)
      @app.call(env)
    end

    private

    def rewrite_request(env)
      url = env[:url]
      user = url.user
      pass = url.password
      return unless user && pass

      # Nullify the user and password components of the url
      # We don't want to send them plain
      url.user = nil
      url.password = nil

      # Set the Authorization header
      value = Base64.encode64([user, pass].join(':')).gsub("\n", '')
      env.request_headers[KEY] = "Basic #{value}"
    end
  end
end

basic_auth_overwrite_spec.rb

require 'faraday_middleware/basic_auth_overwrite'

module FaradayMiddleware
  RSpec.describe BasicAuthOverwrite do

    def new_client(method)
      Faraday.new do |builder|
        builder.use described_class
        builder.adapter :test do |stub|
          stub.send(method, '/') { |env|
            [200, {}, env.request_headers['Authorization']]
          }
        end
      end
    end

    context 'when url contains username and password' do
      it 'adds Authorization headers' do
        client = new_client(:post)
        result = client.post('http://user:password@example.com/')

        expect(result.env[:url]).to eq(URI('http://example.com/'))

        encoded_credentials = result.body.sub('Basic ', '')
        expect(Base64.decode64(encoded_credentials)).to eq('user:password')
      end
    end

    context 'when url does not contain credentials' do
      it 'does not add authorization headers' do
        client = new_client(:post)
        result = client.post('http://example.com/')

        expect(result.env[:url]).to eq(URI('http://example.com/'))
        expect(result.body).to be_nil
      end
    end
  end
end