Log a warning when cluster admins enable Insecure TLS Ciphers for kubelet or apiserver

cmd/kubelet/app/options/options.go

@@ -464,11 +464,13 @@ func AddKubeletConfigFlags(mainfs *pflag.FlagSet, c *kubeletconfig.KubeletConfig
 	fs.StringVar(&c.TLSPrivateKeyFile, "tls-private-key-file", c.TLSPrivateKeyFile, "File containing x509 private key matching --tls-cert-file.")
 	fs.BoolVar(&c.ServerTLSBootstrap, "rotate-server-certificates", c.ServerTLSBootstrap, "Auto-request and rotate the kubelet serving certificates by requesting new certificates from the kube-apiserver when the certificate expiration approaches. Requires the RotateKubeletServerCertificate feature gate to be enabled, and approval of the submitted CertificateSigningRequest objects.")
 
-	tlsCipherPossibleValues := cliflag.TLSCipherPossibleValues()
+	tlsCipherPreferredValues := cliflag.PreferredTLSCipherNames()
+	tlsCipherInsecureValues := cliflag.InsecureTLSCipherNames()
 	fs.StringSliceVar(&c.TLSCipherSuites, "tls-cipher-suites", c.TLSCipherSuites,
 		"Comma-separated list of cipher suites for the server. "+
-			"If omitted, the default Go cipher suites will be used. "+
-			"Possible values: "+strings.Join(tlsCipherPossibleValues, ","))
+			"If omitted, the default Go cipher suites will be used. \n"+
+			"Preferred values: "+strings.Join(tlsCipherPreferredValues, ", ")+". \n"+
+			"Insecure values: "+strings.Join(tlsCipherInsecureValues, ", ")+".")
 	tlsPossibleVersions := cliflag.TLSPossibleVersions()
 	fs.StringVar(&c.TLSMinVersion, "tls-min-version", c.TLSMinVersion,
 		"Minimum TLS version supported. "+

cmd/kubelet/app/server.go

@@ -60,6 +60,7 @@ import (
 	"k8s.io/client-go/util/connrotation"
 	"k8s.io/client-go/util/keyutil"
 	cloudprovider "k8s.io/cloud-provider"
+	"k8s.io/component-base/cli/flag"
 	cliflag "k8s.io/component-base/cli/flag"
 	"k8s.io/component-base/configz"
 	"k8s.io/component-base/featuregate"
@@ -1010,6 +1011,17 @@ func InitializeTLS(kf *options.KubeletFlags, kc *kubeletconfiginternal.KubeletCo
 		return nil, err
 	}
 
+	if len(tlsCipherSuites) > 0 {
+		insecureCiphers := flag.InsecureTLSCiphers()
+		for i := 0; i < len(tlsCipherSuites); i++ {
+			for cipherName, cipherID := range insecureCiphers {
+				if tlsCipherSuites[i] == cipherID {
+					klog.Warningf("Use of insecure cipher '%s' detected.", cipherName)
+				}
+			}
+		}
+	}
+
 	minTLSVersion, err := cliflag.TLSVersion(kc.TLSMinVersion)
 	if err != nil {
 		return nil, err

staging/src/k8s.io/apiserver/pkg/server/BUILD

@@ -117,6 +117,7 @@ go_library(
         "//staging/src/k8s.io/apiserver/pkg/util/openapi:go_default_library",
         "//staging/src/k8s.io/client-go/informers:go_default_library",
         "//staging/src/k8s.io/client-go/rest:go_default_library",
+        "//staging/src/k8s.io/component-base/cli/flag:go_default_library",
         "//staging/src/k8s.io/component-base/logs:go_default_library",
         "//vendor/github.com/coreos/go-systemd/daemon:go_default_library",
         "//vendor/github.com/emicklei/go-restful:go_default_library",

staging/src/k8s.io/apiserver/pkg/server/options/serving.go

@@ -171,11 +171,13 @@ func (s *SecureServingOptions) AddFlags(fs *pflag.FlagSet) {
 	fs.StringVar(&s.ServerCert.CertKey.KeyFile, "tls-private-key-file", s.ServerCert.CertKey.KeyFile,
 		"File containing the default x509 private key matching --tls-cert-file.")
 
-	tlsCipherPossibleValues := cliflag.TLSCipherPossibleValues()
+	tlsCipherPreferredValues := cliflag.PreferredTLSCipherNames()
+	tlsCipherInsecureValues := cliflag.InsecureTLSCipherNames()
 	fs.StringSliceVar(&s.CipherSuites, "tls-cipher-suites", s.CipherSuites,
 		"Comma-separated list of cipher suites for the server. "+
-			"If omitted, the default Go cipher suites will be use.  "+
-			"Possible values: "+strings.Join(tlsCipherPossibleValues, ","))
+			"If omitted, the default Go cipher suites will be used. \n"+
+			"Preferred values: "+strings.Join(tlsCipherPreferredValues, ", ")+". \n"+
+			"Insecure values: "+strings.Join(tlsCipherInsecureValues, ", ")+".")
 
 	tlsPossibleVersions := cliflag.TLSPossibleVersions()
 	fs.StringVar(&s.MinTLSVersion, "tls-min-version", s.MinTLSVersion,

staging/src/k8s.io/apiserver/pkg/server/secure_serving.go

@@ -25,6 +25,7 @@ import (
 	"time"
 
 	"golang.org/x/net/http2"
+	"k8s.io/component-base/cli/flag"
 	"k8s.io/klog/v2"
 
 	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
@@ -56,6 +57,14 @@ func (s *SecureServingInfo) tlsConfig(stopCh <-chan struct{}) (*tls.Config, erro
 	}
 	if len(s.CipherSuites) > 0 {
 		tlsConfig.CipherSuites = s.CipherSuites
+		insecureCiphers := flag.InsecureTLSCiphers()
+		for i := 0; i < len(s.CipherSuites); i++ {
+			for cipherName, cipherID := range insecureCiphers {
+				if s.CipherSuites[i] == cipherID {
+					klog.Warningf("Use of insecure cipher '%s' detected.", cipherName)
+				}
+			}
+		}
 	}
 
 	if s.ClientCA != nil {

staging/src/k8s.io/component-base/cli/flag/ciphersuites_flag.go

@@ -25,23 +25,18 @@ import (
 
 // ciphers maps strings into tls package cipher constants in
 // https://golang.org/pkg/crypto/tls/#pkg-constants
+// to be replaced by tls.CipherSuites() when the project migrates to go1.14.
 var ciphers = map[string]uint16{
-	"TLS_RSA_WITH_RC4_128_SHA":                tls.TLS_RSA_WITH_RC4_128_SHA,
 	"TLS_RSA_WITH_3DES_EDE_CBC_SHA":           tls.TLS_RSA_WITH_3DES_EDE_CBC_SHA,
 	"TLS_RSA_WITH_AES_128_CBC_SHA":            tls.TLS_RSA_WITH_AES_128_CBC_SHA,
 	"TLS_RSA_WITH_AES_256_CBC_SHA":            tls.TLS_RSA_WITH_AES_256_CBC_SHA,
-	"TLS_RSA_WITH_AES_128_CBC_SHA256":         tls.TLS_RSA_WITH_AES_128_CBC_SHA256,
 	"TLS_RSA_WITH_AES_128_GCM_SHA256":         tls.TLS_RSA_WITH_AES_128_GCM_SHA256,
 	"TLS_RSA_WITH_AES_256_GCM_SHA384":         tls.TLS_RSA_WITH_AES_256_GCM_SHA384,
-	"TLS_ECDHE_ECDSA_WITH_RC4_128_SHA":        tls.TLS_ECDHE_ECDSA_WITH_RC4_128_SHA,
 	"TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA":    tls.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA,
 	"TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA":    tls.TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA,
-	"TLS_ECDHE_RSA_WITH_RC4_128_SHA":          tls.TLS_ECDHE_RSA_WITH_RC4_128_SHA,
 	"TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA":     tls.TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA,
 	"TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA":      tls.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA,
 	"TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA":      tls.TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA,
-	"TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256": tls.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256,
-	"TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256":   tls.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256,
 	"TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256":   tls.TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
 	"TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256": tls.TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
 	"TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384":   tls.TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
@@ -53,21 +48,76 @@ var ciphers = map[string]uint16{
 	"TLS_AES_256_GCM_SHA384":                  tls.TLS_AES_256_GCM_SHA384,
 }
 
-func TLSCipherPossibleValues() []string {
+// to be replaced by tls.InsecureCipherSuites() when the project migrates to go1.14.
+var insecureCiphers = map[string]uint16{
+	"TLS_RSA_WITH_RC4_128_SHA":                tls.TLS_RSA_WITH_RC4_128_SHA,
+	"TLS_RSA_WITH_AES_128_CBC_SHA256":         tls.TLS_RSA_WITH_AES_128_CBC_SHA256,
+	"TLS_ECDHE_ECDSA_WITH_RC4_128_SHA":        tls.TLS_ECDHE_ECDSA_WITH_RC4_128_SHA,
+	"TLS_ECDHE_RSA_WITH_RC4_128_SHA":          tls.TLS_ECDHE_RSA_WITH_RC4_128_SHA,
+	"TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256": tls.TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256,
+	"TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256":   tls.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256,
+}
+
+// InsecureTLSCiphers returns the cipher suites implemented by crypto/tls which have
+// security issues.
+func InsecureTLSCiphers() map[string]uint16 {
+	cipherKeys := make(map[string]uint16, len(insecureCiphers))
+	for k, v := range insecureCiphers {
+		cipherKeys[k] = v
+	}
+	return cipherKeys
+}
+
+// InsecureTLSCipherNames returns a list of cipher suite names implemented by crypto/tls
+// which have security issues.
+func InsecureTLSCipherNames() []string {
+	cipherKeys := sets.NewString()
+	for key := range insecureCiphers {
+		cipherKeys.Insert(key)
+	}
+	return cipherKeys.List()
+}
+
+// PreferredTLSCipherNames returns a list of cipher suite names implemented by crypto/tls.
+func PreferredTLSCipherNames() []string {
 	cipherKeys := sets.NewString()
 	for key := range ciphers {
 		cipherKeys.Insert(key)
 	}
 	return cipherKeys.List()
 }
 
+func allCiphers() map[string]uint16 {
+	acceptedCiphers := make(map[string]uint16, len(ciphers)+len(insecureCiphers))
+	for k, v := range ciphers {
+		acceptedCiphers[k] = v
+	}
+	for k, v := range insecureCiphers {
+		acceptedCiphers[k] = v
+	}
+	return acceptedCiphers
+}
+
+// TLSCipherPossibleValues returns all acceptable cipher suite names.
+// This is a combination of both InsecureTLSCipherNames() and PreferredTLSCipherNames().
+func TLSCipherPossibleValues() []string {
+	cipherKeys := sets.NewString()
+	acceptedCiphers := allCiphers()
+	for key := range acceptedCiphers {
+		cipherKeys.Insert(key)
+	}
+	return cipherKeys.List()
+}
+
+// TLSCipherSuites returns a list of cipher suite IDs from the cipher suite names passed.
 func TLSCipherSuites(cipherNames []string) ([]uint16, error) {
 	if len(cipherNames) == 0 {
 		return nil, nil
 	}
 	ciphersIntSlice := make([]uint16, 0)
+	possibleCiphers := allCiphers()
 	for _, cipher := range cipherNames {
-		intValue, ok := ciphers[cipher]
+		intValue, ok := possibleCiphers[cipher]
 		if !ok {
 			return nil, fmt.Errorf("Cipher suite %s not supported or doesn't exist", cipher)
 		}
@@ -83,6 +133,7 @@ var versions = map[string]uint16{
 	"VersionTLS13": tls.VersionTLS13,
 }
 
+// TLSPossibleVersions returns all acceptable values for TLS Version.
 func TLSPossibleVersions() []string {
 	versionsKeys := sets.NewString()
 	for key := range versions {
@@ -91,6 +142,7 @@ func TLSPossibleVersions() []string {
 	return versionsKeys.List()
 }
 
+// TLSVersion returns the TLS Version ID for the version name passed.
 func TLSVersion(versionName string) (uint16, error) {
 	if len(versionName) == 0 {
 		return DefaultTLSVersion(), nil
@@ -101,6 +153,7 @@ func TLSVersion(versionName string) (uint16, error) {
 	return 0, fmt.Errorf("unknown tls version %q", versionName)
 }
 
+// DefaultTLSVersion defines the default TLS Version.
 func DefaultTLSVersion() uint16 {
 	// Can't use SSLv3 because of POODLE and BEAST
 	// Can't use TLSv1.0 because of POODLE and BEAST using CBC cipher

staging/src/k8s.io/component-base/cli/flag/ciphersuites_flag_test.go

@@ -86,18 +86,18 @@ func TestConstantMaps(t *testing.T) {
 		if strings.HasPrefix(declName, "VersionTLS") {
 			discoveredVersions[declName] = true
 		}
-		if strings.HasPrefix(declName, "TLS_RSA_") || strings.HasPrefix(declName, "TLS_ECDHE_") ||
-			strings.HasPrefix(declName, "TLS_AES_") || strings.HasPrefix(declName, "TLS_CHACHA20_") {
+		if strings.HasPrefix(declName, "TLS_") && !strings.HasPrefix(declName, "TLS_FALLBACK_") {
 			discoveredCiphers[declName] = true
 		}
 	}
 
+	acceptedCiphers := allCiphers()
 	for k := range discoveredCiphers {
-		if _, ok := ciphers[k]; !ok {
+		if _, ok := acceptedCiphers[k]; !ok {
 			t.Errorf("discovered cipher tls.%s not in ciphers map", k)
 		}
 	}
-	for k := range ciphers {
+	for k := range acceptedCiphers {
 		if _, ok := discoveredCiphers[k]; !ok {
 			t.Errorf("ciphers map has %s not in tls package", k)
 		}