[v13] Stop users from deleting their last passwordless device (#35856)

* Stop users from deleting their last passwordless device (#35794)

* Fix RegisterTestDevice behavior when using WithPasswordless

lib/auth/auth.go

@@ -2991,8 +2991,13 @@ func (a *Server) DeleteMFADeviceSync(ctx context.Context, req *proto.DeleteMFADe
 	return trace.Wrap(err)
 }
 
-// deleteMFADeviceSafely deletes the user's mfa device while preventing users from deleting their last device
-// for clusters that require second factors, which prevents users from being locked out of their account.
+// deleteMFADeviceSafely deletes the user's mfa device while preventing users
+// from locking themselves out of their account.
+//
+// Deletes are not allowed in the following situations:
+//   - Last MFA device when the cluster requires MFA
+//   - Last resident key credential in a passwordless-capable cluster (avoids
+//     passwordless users from locking themselves out).
 func (a *Server) deleteMFADeviceSafely(ctx context.Context, user, deviceName string) (*types.MFADevice, error) {
 	devs, err := a.Services.GetMFADevices(ctx, user, true)
 	if err != nil {
@@ -3012,6 +3017,11 @@ func (a *Server) deleteMFADeviceSafely(ctx context.Context, user, deviceName str
 	sfToCount := make(map[constants.SecondFactorType]int)
 	var knownDevices int
 	var deviceToDelete *types.MFADevice
+	var numResidentKeys int
+
+	isResidentKey := func(d *types.MFADevice) bool {
+		return d.GetWebauthn() != nil && d.GetWebauthn().ResidentKey
+	}
 
 	// Find the device to delete and count devices.
 	for _, d := range devs {
@@ -3031,6 +3041,10 @@ func (a *Server) deleteMFADeviceSafely(ctx context.Context, user, deviceName str
 
 		sfToCount[sf]++
 		knownDevices++
+
+		if isResidentKey(d) {
+			numResidentKeys++
+		}
 	}
 	if deviceToDelete == nil {
 		return nil, trace.NotFound("MFA device %q does not exist", deviceName)
@@ -3054,6 +3068,20 @@ func (a *Server) deleteMFADeviceSafely(ctx context.Context, user, deviceName str
 		return nil, trace.BadParameter("unexpected second factor type: %s", sf)
 	}
 
+	// Stop users from deleting their last resident key. This prevents
+	// passwordless users from locking themselves out, at the cost of not letting
+	// regular users do it either.
+	//
+	// A better logic would be to apply this only to passwordless users, but we
+	// cannot distinguish users in that manner.
+	// See https://github.com/gravitational/teleport/issues/13219#issuecomment-1148255979.
+	//
+	// TODO(codingllama): Check if the last login type used was passwordless, if
+	//  not then we could let this device be deleted.
+	if authPref.GetAllowPasswordless() && numResidentKeys == 1 && isResidentKey(deviceToDelete) {
+		return nil, trace.BadParameter("cannot delete last passwordless credential for user")
+	}
+
 	if err := a.DeleteMFADevice(ctx, user, deviceToDelete.Id); err != nil {
 		return nil, trace.Wrap(err)
 	}

lib/auth/grpcserver_test.go

@@ -293,6 +293,19 @@ func TestMFADeviceManagement(t *testing.T) {
 		})
 	}
 
+	// Register a 2nd passwordless device, so we can test removal of the
+	// 2nd-to-last resident credential.
+	// This is already tested above so we just use RegisterTestDevice here.
+	const pwdless2DevName = "pwdless2"
+	pwdless2Dev, err := RegisterTestDevice(ctx,
+		cl,
+		pwdless2DevName,
+		proto.DeviceType_DEVICE_TYPE_WEBAUTHN,
+		&TestDevice{Key: devs.WebKey},
+		WithPasswordless(),
+	)
+	require.NoError(t, err, "RegisterTestDevice")
+
 	// Check that all new devices are registered.
 	resp, err = cl.GetMFADevices(ctx, &proto.GetMFADevicesRequest{})
 	require.NoError(t, err)
@@ -303,7 +316,7 @@ func TestMFADeviceManagement(t *testing.T) {
 		deviceIDs[dev.GetName()] = dev.Id
 	}
 	sort.Strings(deviceNames)
-	require.Equal(t, deviceNames, []string{pwdlessDevName, devs.TOTPName, devs.WebName, webDev2Name})
+	require.Equal(t, []string{pwdlessDevName, pwdless2DevName, devs.TOTPName, devs.WebName, webDev2Name}, deviceNames)
 
 	// Delete several of the MFA devices.
 	deleteTests := []struct {
@@ -388,6 +401,22 @@ func TestMFADeviceManagement(t *testing.T) {
 				checkErr:    require.NoError,
 			},
 		},
+		{
+			desc: "delete last passwordless device fails",
+			opts: mfaDeleteTestOpts{
+				initReq: &proto.DeleteMFADeviceRequestInit{
+					DeviceName: pwdless2DevName,
+				},
+				authHandler: devs.webAuthHandler,
+				checkErr: func(t require.TestingT, err error, _ ...interface{}) {
+					require.ErrorContains(t,
+						err,
+						"last passwordless credential",
+						"Unexpected error deleting last passwordless device",
+					)
+				},
+			},
+		},
 		{
 			desc: "delete webauthn device by name",
 			opts: mfaDeleteTestOpts{
@@ -424,7 +453,34 @@ func TestMFADeviceManagement(t *testing.T) {
 		})
 	}
 
-	// Check the remaining number of devices
+	t.Run("delete last passwordless device", func(t *testing.T) {
+		authPref, err := srv.Auth().GetAuthPreference(ctx)
+		require.NoError(t, err, "GetAuthPreference")
+
+		// Deleting the last passwordless device is only allowed if passwordless is
+		// off, so let's do that.
+		authPref.SetAllowPasswordless(false)
+		require.NoError(t, srv.Auth().SetAuthPreference(ctx, authPref), "SetAuthPreference")
+
+		defer func() {
+			authPref.SetAllowPasswordless(true)
+			assert.NoError(t, srv.Auth().SetAuthPreference(ctx, authPref), "Resetting AuthPreference")
+		}()
+
+		testDeleteMFADevice(ctx, t, cl, mfaDeleteTestOpts{
+			initReq: &proto.DeleteMFADeviceRequestInit{
+				DeviceName: pwdless2DevName,
+			},
+			authHandler: func(t *testing.T, c *proto.MFAAuthenticateChallenge) *proto.MFAAuthenticateResponse {
+				resp, err := pwdless2Dev.SolveAuthn(c)
+				require.NoError(t, err, "SolveAuthn")
+				return resp
+			},
+			checkErr: require.NoError,
+		})
+	})
+
+	// Check no remaining devices.
 	resp, err = cl.GetMFADevices(ctx, &proto.GetMFADevicesRequest{})
 	require.NoError(t, err)
 	require.Empty(t, resp.Devices)

lib/auth/helpers_mfa.go

@@ -103,12 +103,18 @@ func (d *TestDevice) registerStream(
 		return trace.Wrap(err)
 	}
 
+	var usage proto.DeviceUsage
+	if d.passwordless {
+		usage = proto.DeviceUsage_DEVICE_USAGE_PASSWORDLESS
+	}
+
 	// Inform device name and type.
 	if err := stream.Send(&proto.AddMFADeviceRequest{
 		Request: &proto.AddMFADeviceRequest_Init{
 			Init: &proto.AddMFADeviceRequestInit{
-				DeviceName: devName,
-				DeviceType: devType,
+				DeviceName:  devName,
+				DeviceType:  devType,
+				DeviceUsage: usage,
 			},
 		},
 	}); err != nil {