core/kubernetes: fix impersonate group header (#5090)

* core/kubernetes: fix impersonate group header

* formatting

authorize/evaluator/headers_evaluator_test.go

@@ -15,6 +15,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/reflect/protoreflect"
 	"google.golang.org/protobuf/types/known/structpb"
 	"google.golang.org/protobuf/types/known/timestamppb"
 
@@ -22,6 +23,7 @@ import (
 	"github.com/pomerium/pomerium/config"
 	"github.com/pomerium/pomerium/pkg/cryptutil"
 	"github.com/pomerium/pomerium/pkg/grpc/session"
+	"github.com/pomerium/pomerium/pkg/grpc/user"
 	"github.com/pomerium/pomerium/pkg/storage"
 )
 
@@ -58,6 +60,8 @@ func TestNewHeadersRequestFromPolicy_nil(t *testing.T) {
 }
 
 func TestHeadersEvaluator(t *testing.T) {
+	t.Parallel()
+
 	type A = []interface{}
 	type M = map[string]interface{}
 
@@ -231,6 +235,26 @@ func TestHeadersEvaluator(t *testing.T) {
 
 		assert.Equal(t, "", output.Headers.Get("fingerprint"))
 	})
+
+	t.Run("kubernetes", func(t *testing.T) {
+		t.Parallel()
+
+		output, err := eval(t,
+			[]protoreflect.ProtoMessage{
+				&session.Session{Id: "s1", UserId: "u1"},
+				&user.User{Id: "u1", Email: "u1@example.com"},
+			},
+			&HeadersRequest{
+				Issuer:                        "from.example.com",
+				ToAudience:                    "to.example.com",
+				KubernetesServiceAccountToken: "TOKEN",
+				Session:                       RequestSession{ID: "s1"},
+			})
+		require.NoError(t, err)
+		assert.Equal(t, "Bearer TOKEN", output.Headers.Get("Authorization"))
+		assert.Equal(t, "u1@example.com", output.Headers.Get("Impersonate-User"))
+		assert.Empty(t, output.Headers["Impersonate-Group"])
+	})
 }
 
 func decodeJWSPayload(t *testing.T, jws string) []byte {

authorize/evaluator/opa/policy/headers.rego

@@ -174,11 +174,12 @@ signed_jwt := io.jwt.encode_sign(jwt_headers, jwt_payload, data.signing_key)
 
 kubernetes_headers := h if {
 	input.kubernetes_service_account_token != ""
-	h := [
+
+	h := remove_empty_header_values([
 		["Authorization", concat(" ", ["Bearer", input.kubernetes_service_account_token])],
 		["Impersonate-User", jwt_payload_email],
 		["Impersonate-Group", get_header_string_value(jwt_payload_groups)],
-	]
+	])
 } else := []
 
 google_cloud_serverless_authentication_service_account := s if {
@@ -267,3 +268,10 @@ get_header_string_value(obj) := s if {
 } else := s if {
 	s := concat(",", [obj])
 }
+
+remove_empty_header_values(arr) := [[k, v] |
+	some idx
+	k := arr[idx][0]
+	v := arr[idx][1]
+	v != ""
+]