oidc authenticator: allow string value as groups claim

pkg/genericapiserver/options/server_run_options.go

@@ -401,7 +401,7 @@ func (s *ServerRunOptions) AddUniversalFlags(fs *pflag.FlagSet) {
 
 	fs.StringVar(&s.OIDCGroupsClaim, "oidc-groups-claim", "", ""+
 		"If provided, the name of a custom OpenID Connect claim for specifying user groups. "+
-		"The claim value is expected to be an array of strings. This flag is experimental, "+
+		"The claim value is expected to be a string or array of strings. This flag is experimental, "+
 		"please see the authentication documentation for further details.")
 
 	fs.Var(&s.RuntimeConfig, "runtime-config", ""+

plugin/pkg/auth/authenticator/token/oidc/oidc.go

@@ -73,7 +73,7 @@ type OIDCOptions struct {
 
 	// GroupsClaim, if specified, causes the OIDCAuthenticator to try to populate the user's
 	// groups with a ID Token field. If the GrouppClaim field is present in a ID Token the value
-	// must be a list of strings.
+	// must be a string or list of strings.
 	GroupsClaim string
 }
 
@@ -251,10 +251,14 @@ func (a *OIDCAuthenticator) AuthenticateToken(value string) (user.Info, bool, er
 	if a.groupsClaim != "" {
 		groups, found, err := claims.StringsClaim(a.groupsClaim)
 		if err != nil {
-			// Custom claim is present, but isn't an array of strings.
-			return nil, false, fmt.Errorf("custom group claim contains invalid object: %v", err)
-		}
-		if found {
+			// Groups type is present but is not an array of strings, try to decode as a string.
+			group, _, err := claims.StringClaim(a.groupsClaim)
+			if err != nil {
+				// Custom claim is present, but isn't an array of strings or a string.
+				return nil, false, fmt.Errorf("custom group claim contains invalid type: %T", claims[a.groupsClaim])
+			}
+			info.Groups = []string{group}
+		} else if found {
 			info.Groups = groups
 		}
 	}

plugin/pkg/auth/authenticator/token/oidc/oidc_test.go

@@ -33,7 +33,7 @@ import (
 	oidctesting "k8s.io/kubernetes/plugin/pkg/auth/authenticator/token/oidc/testing"
 )
 
-func generateToken(t *testing.T, op *oidctesting.OIDCProvider, iss, sub, aud string, usernameClaim, value, groupsClaim string, groups []string, iat, exp time.Time) string {
+func generateToken(t *testing.T, op *oidctesting.OIDCProvider, iss, sub, aud string, usernameClaim, value, groupsClaim string, groups interface{}, iat, exp time.Time) string {
 	signer := op.PrivKey.Signer()
 	claims := oidc.NewClaims(iss, sub, aud, iat, exp)
 	claims.Add(usernameClaim, value)
@@ -49,15 +49,15 @@ func generateToken(t *testing.T, op *oidctesting.OIDCProvider, iss, sub, aud str
 	return jwt.Encode()
 }
 
-func generateGoodToken(t *testing.T, op *oidctesting.OIDCProvider, iss, sub, aud string, usernameClaim, value, groupsClaim string, groups []string) string {
+func generateGoodToken(t *testing.T, op *oidctesting.OIDCProvider, iss, sub, aud string, usernameClaim, value, groupsClaim string, groups interface{}) string {
 	return generateToken(t, op, iss, sub, aud, usernameClaim, value, groupsClaim, groups, time.Now(), time.Now().Add(time.Hour))
 }
 
-func generateMalformedToken(t *testing.T, op *oidctesting.OIDCProvider, iss, sub, aud string, usernameClaim, value, groupsClaim string, groups []string) string {
+func generateMalformedToken(t *testing.T, op *oidctesting.OIDCProvider, iss, sub, aud string, usernameClaim, value, groupsClaim string, groups interface{}) string {
 	return generateToken(t, op, iss, sub, aud, usernameClaim, value, groupsClaim, groups, time.Now(), time.Now().Add(time.Hour)) + "randombits"
 }
 
-func generateExpiredToken(t *testing.T, op *oidctesting.OIDCProvider, iss, sub, aud string, usernameClaim, value, groupsClaim string, groups []string) string {
+func generateExpiredToken(t *testing.T, op *oidctesting.OIDCProvider, iss, sub, aud string, usernameClaim, value, groupsClaim string, groups interface{}) string {
 	return generateToken(t, op, iss, sub, aud, usernameClaim, value, groupsClaim, groups, time.Now().Add(-2*time.Hour), time.Now().Add(-1*time.Hour))
 }
 
@@ -240,6 +240,24 @@ func TestOIDCAuthentication(t *testing.T) {
 				"",
 			},
 			{
+				// Group claim is a string rather than an array. Map that string to a single group.
+				"email",
+				"groups",
+				generateGoodToken(t, op, srv.URL, "client-foo", "client-foo", "email", "foo@example.com", "groups", "group1"),
+				&user.DefaultInfo{Name: "foo@example.com", Groups: []string{"group1"}},
+				true,
+				"",
+			},
+			{
+				// Group claim is not a string or array of strings. Throw out this as invalid.
+				"email",
+				"groups",
+				generateGoodToken(t, op, srv.URL, "client-foo", "client-foo", "email", "foo@example.com", "groups", 1),
+				nil,
+				false,
+				"custom group claim contains invalid type: float64",
+			},
+			{
 				"sub",
 				"",
 				generateMalformedToken(t, op, srv.URL, "client-foo", "client-foo", "sub", "user-foo", "", nil),