Consider Sec-Fetch-Site header for session auth

This warden strategy is primarily used to allow APIv3 requests
from the browser, which only authenticates using its session cookie.

Since this is susceptible to cross-site-request-forgery, prevention of
CSRF must take place. This was so far only ensured through the usage of
the X-Requested-With header. When a client sent along this header, the
server could know that a CORS-preflight request must have been made and
thus the browser most certainly has validated that the request is valid
according to CORS rules.

However, the header itself is a non-standard header and while some JavaScript
frameworks add it to requests, not all of them do. For us this was practically
visible on the API docs hosted under `/api/docs`.

The solution is to expect the browser to send the Sec-Fetch-Site header with a value
of same-origin. This header can't be set through JavaScript, but only by the browser
and the value "same-origin" ensures that scheme, host and port are the same for requester
and requested endpoint, thus eliminating CSRF concerns. This feature is widely supported by
all major browsers, the last of which was Safari which added support 3 years ago.

We might want to consider dropping the check for X-Requested-With entirely, since it should be
superfluous. For now it was left in place for greater compatibility.

lib_static/open_project/authentication/strategies/warden/session.rb

@@ -50,8 +50,9 @@ module OpenProject
             return true if request.get?
 
             # For all other requests, to mitigate CSRF vectors,
-            # require the frontend header to be present.
-            xml_request_header_set?
+            # require frontend headers to be present or browser indication
+            # that this was a same-origin request
+            xml_request_header_set? || same_origin?
           end
 
           def authenticate!
@@ -64,6 +65,10 @@ module OpenProject
             request.env["HTTP_X_REQUESTED_WITH"] == "XMLHttpRequest"
           end
 
+          def same_origin?
+            request.env["HTTP_SEC_FETCH_SITE"] == "same-origin"
+          end
+
           def user_id
             Hash(session)["user_id"]
           end

spec/requests/api/v3/authentication_spec.rb

@@ -42,6 +42,67 @@ RSpec.describe "API V3 Authentication" do
   end
   let(:resource_metadata) { 'resource_metadata="http://test.host/.well-known/oauth-protected-resource"' }
 
+  describe "session auth" do
+    let(:user) { create(:admin) }
+    let(:session_data) { ActiveSupport::HashWithIndifferentAccess.new(user_id: user.id, updated_at: Time.current) }
+
+    before do
+      # rubocop:disable RSpec/AnyInstance
+      allow_any_instance_of(OpenProject::Authentication::Strategies::Warden::Session)
+        .to receive(:session)
+        .and_return(session_data)
+      # rubocop:enable RSpec/AnyInstance
+    end
+
+    context "when making a GET request" do
+      before do
+        get resource
+      end
+
+      it "authenticates successfully" do
+        expect(last_response).to have_http_status :ok
+      end
+    end
+
+    context "when making a POST request" do
+      let(:add_additional_headers) { nil }
+
+      before do
+        header "Content-Type", "application/json"
+        add_additional_headers
+        post resource, { name: "Test" }.to_json
+      end
+
+      it "is not authenticated" do
+        expect(last_response).to have_http_status :unauthorized
+      end
+
+      context "and when the Sec-Fetch-Site header indicates a same-origin request" do
+        let(:add_additional_headers) { header "Sec-Fetch-Site", "same-origin" }
+
+        it "authenticates successfully" do
+          expect(last_response).to have_http_status :created
+        end
+      end
+
+      context "and when the Sec-Fetch-Site header indicates a cross-site request" do
+        let(:add_additional_headers) { header "Sec-Fetch-Site", "cross-site" }
+
+        it "is not authenticated" do
+          expect(last_response).to have_http_status :unauthorized
+        end
+      end
+
+      context "and when the Sec-Fetch-Site header indicates a same-site request" do
+        let(:add_additional_headers) { header "Sec-Fetch-Site", "same-site" }
+
+        it "is not authenticated" do
+          expect(last_response).to have_http_status :unauthorized
+        end
+      end
+    end
+  end
+
   describe "oauth" do
     let(:oauth_access_token) { "" }
     let(:expected_message) { "You did not provide the correct credentials." }