feat: Add uid_attribute option to control the attribute used for th…

…e user id.

Some SAML 2.0 IdPs use the transient name identifier format. In that case the
name identifier changes for each login, which makes the name identifier
unsuitable as a user identifier.

This patch introduces the `uid_attribute` option, that allows us to use a
SAML 2.0 attribute as the user identifier instead of the name identifier.

Fixes issue #120.

README.md

@@ -135,6 +135,8 @@ The service provider metadata used to ease configuration of the SAML SP in the I
   *Note*: All attributes can also be found in an array under `auth_hash[:extra][:raw_info]`,
   so this setting should only be used to map attributes that are part of the OmniAuth info hash schema.
 
+* `:uid_attribute` - Attribute that uniquely identifies the user. If unset, the name identifier returned by the IdP is used.
+
 * See the `OneLogin::RubySaml::Settings` class in the [Ruby SAML gem](https://github.com/onelogin/ruby-saml) for additional supported options.
 
 ## Devise Integration

lib/omniauth/strategies/saml.rb

@@ -28,6 +28,7 @@ def self.inherited(subclass)
         last_name: ["last_name", "lastname", "lastName"]
       }
       option :slo_default_relay_state
+      option :uid_attribute
 
       def request_phase
         options[:assertion_consumer_service_url] ||= callback_url
@@ -136,7 +137,17 @@ def other_phase
         end
       end
 
-      uid { @name_id }
+      uid do
+        if options.uid_attribute
+          ret = find_attribute_by([options.uid_attribute])
+          if ret.nil?
+            raise OmniAuth::Strategies::SAML::ValidationError.new("SAML response missing '#{options.uid_attribute}' attribute")
+          end
+          ret
+        else
+          @name_id
+        end
+      end
 
       info do
         found_attributes = options.attribute_statements.map do |key, values|

spec/omniauth/strategies/saml_spec.rb

@@ -207,6 +207,31 @@ def post_xml(xml=:example_response, opts = {})
       end
     end
 
+    context "when using custom user id attribute" do
+      before :each do
+        saml_options[:idp_cert_fingerprint] = "3B:82:F1:F5:54:FC:A8:FF:12:B8:4B:B8:16:61:1D:E4:8E:9B:E2:3C"
+        saml_options[:uid_attribute] = "http://schemas.xmlsoap.org/ws/2005/05/identity/claims/emailaddress"
+        post_xml :custom_attributes
+      end
+
+      it "should return user id attribute" do
+        expect(auth_hash[:uid]).to eq("[email protected]")
+      end
+    end
+
+    context "when using custom user id attribute, but it is missing" do
+      before :each do
+        saml_options[:uid_attribute] = "missing_attribute"
+        post_xml
+      end
+
+      it "should fail to authenticate" do
+        should fail_with(:invalid_ticket)
+        expect(last_request.env['omniauth.error']).to be_instance_of(OmniAuth::Strategies::SAML::ValidationError)
+        expect(last_request.env['omniauth.error'].message).to eq("SAML response missing 'missing_attribute' attribute")
+      end
+    end
+
     context "when response is a logout response" do
       before :each do
         saml_options[:issuer] = "https://idp.sso.example.com/metadata/29490"