feat: Adding support for pluggable auth credentials (#437)

lib/googleauth/external_account.rb

@@ -17,6 +17,7 @@
 require "googleauth/credentials_loader"
 require "googleauth/external_account/aws_credentials"
 require "googleauth/external_account/identity_pool_credentials"
+require "googleauth/external_account/pluggable_credentials"
 
 module Google
   # Module Auth provides classes that provide Google-specific authorization
@@ -80,6 +81,9 @@ def make_external_account_credentials user_creds
             unless user_creds[:credential_source][:file].nil? && user_creds[:credential_source][:url].nil?
               return Google::Auth::ExternalAccount::IdentityPoolCredentials.new user_creds
             end
+            unless user_creds[:credential_source][:executable].nil?
+              return Google::Auth::ExternalAccount::PluggableAuthCredentials.new user_creds
+            end
             raise INVALID_EXTERNAL_ACCOUNT_TYPE
           end
         end

lib/googleauth/external_account/external_account_utils.rb

@@ -86,6 +86,17 @@ def normalize_timestamp time
             raise "Invalid time value #{time}"
           end
         end
+
+        def service_account_email
+          return nil if @service_account_impersonation_url.nil?
+          start_idx = @service_account_impersonation_url.rindex "/"
+          end_idx = @service_account_impersonation_url.index ":generateAccessToken"
+          if start_idx != -1 && end_idx != -1 && start_idx < end_idx
+            start_idx += 1
+            return @service_account_impersonation_url[start_idx..end_idx]
+          end
+          nil
+        end
       end
     end
   end

lib/googleauth/external_account/pluggable_credentials.rb

@@ -0,0 +1,156 @@
+# Copyright 2023 Google, Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+require "open3"
+require "time"
+require "googleauth/external_account/base_credentials"
+require "googleauth/external_account/external_account_utils"
+
+module Google
+  # Module Auth provides classes that provide Google-specific authorization used to access Google APIs.
+  module Auth
+    module ExternalAccount
+      # This module handles the retrieval of credentials from Google Cloud by utilizing the any 3PI
+      # provider then exchanging the credentials for a short-lived Google Cloud access token.
+      class PluggableAuthCredentials
+        # constant for pluggable auth enablement in environment variable.
+        ENABLE_PLUGGABLE_ENV = "GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES".freeze
+        EXECUTABLE_SUPPORTED_MAX_VERSION = 1
+        EXECUTABLE_TIMEOUT_MILLIS_DEFAULT = 30 * 1000
+        EXECUTABLE_TIMEOUT_MILLIS_LOWER_BOUND = 5 * 1000
+        EXECUTABLE_TIMEOUT_MILLIS_UPPER_BOUND = 120 * 1000
+        ID_TOKEN_TYPE = ["urn:ietf:params:oauth:token-type:jwt", "urn:ietf:params:oauth:token-type:id_token"].freeze
+
+        include Google::Auth::ExternalAccount::BaseCredentials
+        include Google::Auth::ExternalAccount::ExternalAccountUtils
+        extend CredentialsLoader
+
+        # Will always be nil, but method still gets used.
+        attr_reader :client_id
+
+        # Initialize from options map.
+        #
+        # @param [string] audience
+        # @param [hash{symbol => value}] credential_source
+        #     credential_source is a hash that contains either source file or url.
+        #     credential_source_format is either text or json. To define how we parse the credential response.
+        #
+        def initialize options = {}
+          base_setup options
+
+          @audience = options[:audience]
+          @credential_source = options[:credential_source] || {}
+          @credential_source_executable = @credential_source[:executable]
+          raise "Missing excutable source. An 'executable' must be provided" if @credential_source_executable.nil?
+          @credential_source_executable_command = @credential_source_executable[:command]
+          if @credential_source_executable_command.nil?
+            raise "Missing command field. Executable command must be provided."
+          end
+          @credential_source_executable_timeout_millis = @credential_source_executable[:timeout_millis] ||
+                                                         EXECUTABLE_TIMEOUT_MILLIS_DEFAULT
+          if @credential_source_executable_timeout_millis < EXECUTABLE_TIMEOUT_MILLIS_LOWER_BOUND ||
+             @credential_source_executable_timeout_millis > EXECUTABLE_TIMEOUT_MILLIS_UPPER_BOUND
+            raise "Timeout must be between 5 and 120 seconds."
+          end
+          @credential_source_executable_output_file = @credential_source_executable[:output_file]
+        end
+
+        def retrieve_subject_token!
+          unless ENV[ENABLE_PLUGGABLE_ENV] == "1"
+            raise "Executables need to be explicitly allowed (set GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES to '1') " \
+                  "to run."
+          end
+          # check output file first
+          subject_token = load_subject_token_from_output_file
+          return subject_token unless subject_token.nil?
+          # environment variable injection
+          env = inject_environment_variables
+          output = subprocess_with_timeout env, @credential_source_executable_command,
+                                           @credential_source_executable_timeout_millis
+          response = MultiJson.load output, symbolize_keys: true
+          parse_subject_token response
+        end
+
+        private
+
+        def load_subject_token_from_output_file
+          return nil if @credential_source_executable_output_file.nil?
+          return nil unless File.exist? @credential_source_executable_output_file
+          begin
+            content = File.read @credential_source_executable_output_file, encoding: "utf-8"
+            response = MultiJson.load content, symbolize_keys: true
+          rescue StandardError
+            return nil
+          end
+          begin
+            subject_token = parse_subject_token response
+          rescue StandardError => e
+            return nil if e.message.match(/The token returned by the executable is expired/)
+            raise e
+          end
+          subject_token
+        end
+
+        def parse_subject_token response
+          validate_response_schema response
+          unless response[:success]
+            if response[:code].nil? || response[:message].nil?
+              raise "Error code and message fields are required in the response."
+            end
+            raise "Executable returned unsuccessful response: code: #{response[:code]}, message: #{response[:message]}."
+          end
+          if response[:expiration_time] && response[:expiration_time] < Time.now.to_i
+            raise "The token returned by the executable is expired."
+          end
+          raise "The executable response is missing the token_type field." if response[:token_type].nil?
+          return response[:id_token] if ID_TOKEN_TYPE.include? response[:token_type]
+          return response[:saml_response] if response[:token_type] == "urn:ietf:params:oauth:token-type:saml2"
+          raise "Executable returned unsupported token type."
+        end
+
+        def validate_response_schema response
+          raise "The executable response is missing the version field." if response[:version].nil?
+          if response[:version] > EXECUTABLE_SUPPORTED_MAX_VERSION
+            raise "Executable returned unsupported version #{response[:version]}."
+          end
+          raise "The executable response is missing the success field." if response[:success].nil?
+        end
+
+        def inject_environment_variables
+          env = ENV.to_h
+          env["GOOGLE_EXTERNAL_ACCOUNT_AUDIENCE"] = @audience
+          env["GOOGLE_EXTERNAL_ACCOUNT_TOKEN_TYPE"] = @subject_token_type
+          env["GOOGLE_EXTERNAL_ACCOUNT_INTERACTIVE"] = "0" # only non-interactive mode we support.
+          unless @service_account_impersonation_url.nil?
+            env["GOOGLE_EXTERNAL_ACCOUNT_IMPERSONATED_EMAIL"] = service_account_email
+          end
+          unless @credential_source_executable_output_file.nil?
+            env["GOOGLE_EXTERNAL_ACCOUNT_OUTPUT_FILE"] = @credential_source_executable_output_file
+          end
+          env
+        end
+
+        def subprocess_with_timeout environment_vars, command, timeout_seconds
+          Timeout.timeout timeout_seconds do
+            output, error, status = Open3.capture3 environment_vars, command
+            unless status.success?
+              raise "Executable exited with non-zero return code #{status.exitstatus}. Error: #{output}, #{error}"
+            end
+            output
+          end
+        end
+      end
+    end
+  end
+end