Validate that SAML requests are originated from gitlab

If the request wasn't initiated by gitlab we shouldn't add the new
identity to the user, and instead show that we weren't able to link
the identity to the user.

This should fix: https://gitlab.com/gitlab-org/gitlab-ce/issues/56509

3 files changed, 68 insertions, 2 deletions

diff --git a/lib/gitlab/auth/omniauth_identity_linker_base.rb b/lib/gitlab/auth/omniauth_identity_linker_base.rb
index c620fc5d6bd..a6c247f31a7 100644
--- a/lib/gitlab/auth/omniauth_identity_linker_base.rb
+++ b/lib/gitlab/auth/omniauth_identity_linker_base.rb
@@ -3,12 +3,13 @@
 module Gitlab
   module Auth
     class OmniauthIdentityLinkerBase
-      attr_reader :current_user, :oauth
+      attr_reader :current_user, :oauth, :session
 
-      def initialize(current_user, oauth)
+      def initialize(current_user, oauth, session = {})
         @current_user = current_user
         @oauth = oauth
         @changed = false
+        @session = session
       end
 
       def link
diff --git a/lib/gitlab/auth/saml/identity_linker.rb b/lib/gitlab/auth/saml/identity_linker.rb
index ae0d6dded4e..93195c3189f 100644
--- a/lib/gitlab/auth/saml/identity_linker.rb
+++ b/lib/gitlab/auth/saml/identity_linker.rb
@@ -4,6 +4,30 @@ module Gitlab
   module Auth
     module Saml
       class IdentityLinker < OmniauthIdentityLinkerBase
+        extend ::Gitlab::Utils::Override
+
+        UnverifiedRequest = Class.new(StandardError)
+
+        override :link
+        def link
+          raise_unless_request_is_gitlab_initiated! if unlinked?
+
+          super
+        end
+
+        protected
+
+        def raise_unless_request_is_gitlab_initiated!
+          raise UnverifiedRequest unless valid_gitlab_initiated_request?
+        end
+
+        def valid_gitlab_initiated_request?
+          OriginValidator.new(session).gitlab_initiated?(saml_response)
+        end
+
+        def saml_response
+          oauth.fetch(:extra, {}).fetch(:response_object, {})
+        end
       end
     end
   end
diff --git a/lib/gitlab/auth/saml/origin_validator.rb b/lib/gitlab/auth/saml/origin_validator.rb
new file mode 100644
index 00000000000..4ecc688888f
--- /dev/null
+++ b/lib/gitlab/auth/saml/origin_validator.rb
@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Auth
+    module Saml
+      class OriginValidator
+        AUTH_REQUEST_SESSION_KEY = "last_authn_request_id".freeze
+
+        def initialize(session)
+          @session = session || {}
+        end
+
+        def store_origin(authn_request)
+          session[AUTH_REQUEST_SESSION_KEY] = authn_request.uuid
+        end
+
+        def gitlab_initiated?(saml_response)
+          return false if identity_provider_initiated?(saml_response)
+
+          matches?(saml_response)
+        end
+
+        private
+
+        attr_reader :session
+
+        def matches?(saml_response)
+          saml_response.in_response_to == expected_request_id
+        end
+
+        def identity_provider_initiated?(saml_response)
+          saml_response.in_response_to.blank?
+        end
+
+        def expected_request_id
+          session[AUTH_REQUEST_SESSION_KEY]
+        end
+      end
+    end
+  end
+end