Add --block-instance-metadata flag (#396)

* Add --block-instance-metadata flag

* Switch from 404 to 403 for blocked requests

* Only block /metadata/instance

* Add message body to 403 response

* Add docs on --block-instance-metadata

cmd/nmi/main.go

@@ -41,6 +41,7 @@ var (
 	findIdentityRetryIntervalInSeconds = pflag.Int("find-identity-retry-interval", defaultlistPodIDsRetryIntervalInSeconds, "Retry interval to find assigned identities in seconds")
 	enableProfile                      = pflag.Bool("enableProfile", false, "Enable/Disable pprof profiling")
 	enableScaleFeatures                = pflag.Bool("enableScaleFeatures", false, "Enable/Disable features for scale clusters")
+	blockInstanceMetadata              = pflag.Bool("block-instance-metadata", false, "Block instance metadata endpoints")
 )
 
 func main() {
@@ -75,7 +76,7 @@ func main() {
 	exit := make(<-chan struct{})
 	client.Start(exit)
 	*forceNamespaced = *forceNamespaced || "true" == os.Getenv("FORCENAMESPACED")
-	s := server.NewServer(*forceNamespaced, *micNamespace)
+	s := server.NewServer(*forceNamespaced, *micNamespace, *blockInstanceMetadata)
 	s.KubeClient = client
 	s.MetadataIP = *metadataIP
 	s.MetadataPort = *metadataPort

docs/readmes/README.featureflags.md

@@ -21,3 +21,24 @@ node/VMSS.
 
 Aad-pod-identity has a new flag clientQps which can be used to control the total number of client operations performed per second
 to the API server by MIC.
+
+## Block Instance Metadata flag
+
+The Azure Metadata API includes endpoints under `/instance/metadata` which
+provide information about the virtual machine. You can see examples of this
+endpoint in [the Azure documentation](https://docs.microsoft.com/en-us/azure/virtual-machines/linux/instance-metadata-service#retrieving-all-metadata-for-an-instance).
+
+Some of the information returned by this endpoint may be considered sensitive
+or secret. The response includes information on the operating system and image,
+tags, resource IDs, network, and VM custom data.
+
+This information is legitimately useful for many use cases, but also presents a
+risk. If an attacker can exploit a vulnerability that allows them to read from
+this endpoint, they may be able to access sensitive information even if the
+vulnerable Pod does not use Managed Identity.
+
+The `blockInstanceMetadata` flag for NMI will intercept any  requests to this
+endpoint from Pods which are not using host networking and return an HTTP 403
+Forbidden response. This flag is disabled by default to maximize compatibility.
+Users are encouraged to determine if this option is relevant and beneficial for
+their use cases.

pkg/nmi/server/server.go

@@ -45,6 +45,7 @@ type Server struct {
 	IsNamespaced                       bool
 	MICNamespace                       string
 	Initialized                        bool
+	BlockInstanceMetadata              bool
 
 	ListPodIDsRetryAttemptsForCreated  int
 	ListPodIDsRetryAttemptsForAssigned int
@@ -58,10 +59,11 @@ type NMIResponse struct {
 }
 
 // NewServer will create a new Server with default values.
-func NewServer(isNamespaced bool, micNamespace string) *Server {
+func NewServer(isNamespaced bool, micNamespace string, blockInstanceMetadata bool) *Server {
 	return &Server{
 		IsNamespaced:          isNamespaced,
 		MICNamespace:          micNamespace,
+		BlockInstanceMetadata: blockInstanceMetadata,
 	}
 }
 
@@ -74,6 +76,9 @@ func (s *Server) Run() error {
 	mux.Handle("/metadata/identity/oauth2/token/", appHandler(s.msiHandler))
 	mux.Handle("/host/token", appHandler(s.hostHandler))
 	mux.Handle("/host/token/", appHandler(s.hostHandler))
+	if s.BlockInstanceMetadata {
+		mux.Handle("/metadata/instance", http.HandlerFunc(forbiddenHandler))
+	}
 	mux.Handle("/", appHandler(s.defaultPathHandler))
 
 	log.Infof("Listening on port %s", s.NMIPort)
@@ -458,6 +463,11 @@ func (s *Server) defaultPathHandler(logger *log.Entry, w http.ResponseWriter, r
 	w.Write(body)
 }
 
+// forbiddenHandler responds to any request with HTTP 403 Forbidden
+func forbiddenHandler(w http.ResponseWriter, r *http.Request) {
+	http.Error(w, "Request blocked by AAD Pod Identity NMI", http.StatusForbidden)
+}
+
 func copyHeader(dst, src http.Header) {
 	for k, vv := range src {
 		for _, v := range vv {