bootstrapper: make Azure auth method configurable on cluster init (#1346)

* bootstrapper: make Azure auth method configurable on cluster init
* azure: convert uami resource ID to clientID


Co-authored-by: 3u13r <lc@edgeless.systems>

bootstrapper/internal/kubernetes/BUILD.bazel

@@ -13,7 +13,6 @@ go_library(
     deps = [
         "//bootstrapper/internal/kubernetes/k8sapi",
         "//bootstrapper/internal/kubernetes/kubewaiter",
-        "//internal/cloud/azureshared",
         "//internal/cloud/cloudprovider",
         "//internal/cloud/gcpshared",
         "//internal/cloud/metadata",

bootstrapper/internal/kubernetes/kubernetes.go

@@ -20,7 +20,6 @@ import (
 
 	"github.com/edgelesssys/constellation/v2/bootstrapper/internal/kubernetes/k8sapi"
 	"github.com/edgelesssys/constellation/v2/bootstrapper/internal/kubernetes/kubewaiter"
-	"github.com/edgelesssys/constellation/v2/internal/cloud/azureshared"
 	"github.com/edgelesssys/constellation/v2/internal/cloud/cloudprovider"
 	"github.com/edgelesssys/constellation/v2/internal/cloud/gcpshared"
 	"github.com/edgelesssys/constellation/v2/internal/cloud/openstack"
@@ -477,24 +476,6 @@ func (k *KubeWrapper) setupExtraVals(ctx context.Context, serviceConfig constell
 			"subnetworkPodCIDR": serviceConfig.subnetworkPodCIDR,
 		}
 
-		subscriptionID, resourceGroup, err := azureshared.BasicsFromProviderID(instance.ProviderID)
-		if err != nil {
-			return nil, err
-		}
-		creds, err := azureshared.ApplicationCredentialsFromURI(serviceConfig.cloudServiceAccountURI)
-		if err != nil {
-			return nil, err
-		}
-
-		extraVals["autoscaler"] = map[string]any{
-			"Azure": map[string]any{
-				"clientID":       creds.AppClientID,
-				"clientSecret":   creds.ClientSecretValue,
-				"resourceGroup":  resourceGroup,
-				"subscriptionID": subscriptionID,
-				"tenantID":       creds.TenantID,
-			},
-		}
 	case cloudprovider.OpenStack:
 		creds, err := openstack.AccountKeyFromURI(serviceConfig.cloudServiceAccountURI)
 		if err != nil {

cli/internal/cmd/init.go

@@ -451,6 +451,9 @@ func (i *initCmd) getMarshaledServiceAccountURI(provider cloudprovider.Provider,
 			AppClientID:       config.Provider.Azure.AppClientID,
 			ClientSecretValue: config.Provider.Azure.ClientSecretValue,
 			Location:          config.Provider.Azure.Location,
+			// TODO(malt3): Switch preferred auth method to uami as planned by AB#2961
+			PreferredAuthMethod: azureshared.AuthMethodServicePrincipal,
+			UamiResourceID:      config.Provider.Azure.UserAssignedIdentity,
 		}
 		return creds.ToCloudServiceAccountURI(), nil
 

cli/internal/helm/BUILD.bazel

@@ -181,7 +181,6 @@ go_library(
         "charts/edgeless/constellation-services/charts/autoscaler/Chart.yaml",
         "charts/edgeless/constellation-services/charts/autoscaler/templates/aws-deployment.yaml",
         "charts/edgeless/constellation-services/charts/autoscaler/templates/azure-deployment.yaml",
-        "charts/edgeless/constellation-services/charts/autoscaler/templates/azure-secret.yaml",
         "charts/edgeless/constellation-services/charts/autoscaler/templates/clusterrole.yaml",
         "charts/edgeless/constellation-services/charts/autoscaler/templates/clusterrolebinding.yaml",
         "charts/edgeless/constellation-services/charts/autoscaler/templates/gcp-deployment.yaml",

cli/internal/helm/charts/edgeless/constellation-services/charts/autoscaler/templates/azure-deployment.yaml

@@ -24,43 +24,25 @@ spec:
         - name: cluster-autoscaler
           image: {{ .Values.image | quote }}
           imagePullPolicy: IfNotPresent
+          command:
+          - ./cluster-autoscaler
+          args:
+          - --cloud-provider=azure
+          - --cloud-config=/etc/azure/azure.json
+          - --logtostderr=true
+          - --namespace=kube-system
+          - --stderrthreshold=info
+          - --v=2
+          volumeMounts:
+          - name: azureconfig
+            mountPath: /etc/azure
+            readOnly: true
           livenessProbe:
             httpGet:
               path: /health-check
               port: 8085
           ports:
             - containerPort: 8085
-          env:
-            - name: ARM_SUBSCRIPTION_ID
-              valueFrom:
-                secretKeyRef:
-                  key: SubscriptionID
-                  name: cluster-autoscaler-azure
-            - name: ARM_RESOURCE_GROUP
-              valueFrom:
-                secretKeyRef:
-                  key: ResourceGroup
-                  name: cluster-autoscaler-azure
-            - name: ARM_TENANT_ID
-              valueFrom:
-                secretKeyRef:
-                  key: TenantID
-                  name: cluster-autoscaler-azure
-            - name: ARM_CLIENT_ID
-              valueFrom:
-                secretKeyRef:
-                  key: ClientID
-                  name: cluster-autoscaler-azure
-            - name: ARM_CLIENT_SECRET
-              valueFrom:
-                secretKeyRef:
-                  key: ClientSecret
-                  name: cluster-autoscaler-azure
-            - name: ARM_VM_TYPE
-              valueFrom:
-                secretKeyRef:
-                  key: VMType
-                  name: cluster-autoscaler-azure
           resources: {}
       dnsPolicy: ClusterFirst
       nodeSelector:
@@ -78,4 +60,8 @@ spec:
           key: node.cloudprovider.kubernetes.io/uninitialized
           operator: Equal
           value: "true"
+      volumes:
+        - name: azureconfig
+          secret:
+            secretName: azureconfig
 {{- end -}}

cli/internal/helm/charts/edgeless/constellation-services/charts/autoscaler/templates/azure-secret.yaml

@@ -1,15 +0,0 @@
-{{- if eq .Values.csp "Azure" -}}
-apiVersion: v1
-kind: Secret
-metadata:
-  name: cluster-autoscaler-azure
-  namespace: {{ .Release.Namespace }}
-data:
-  ClientID: {{ .Values.Azure.clientID | b64enc }}
-  ClientSecret: {{ .Values.Azure.clientSecret | b64enc }}
-  ResourceGroup: {{ .Values.Azure.resourceGroup | b64enc }}
-  SubscriptionID: {{ .Values.Azure.subscriptionID | b64enc }}
-  TenantID: {{ .Values.Azure.tenantID | b64enc }}
-  {{/* b64encode("vmss") */}}
-  VMType: dm1zcw==
-{{- end -}}

cli/internal/helm/charts/edgeless/constellation-services/charts/autoscaler/values.schema.json

@@ -16,60 +16,12 @@
             "examples": [
                 "registry.k8s.io/autoscaling/cluster-autoscaler:v1.23.1"
             ]
-        },
-        "Azure": {
-            "description": "Config values required for deployment on Azure",
-            "type": "object",
-            "properties": {
-                "clientID": {
-                    "description": "Client ID of the service account used to access the Azure API.",
-                    "type": "string"
-                },
-                "clientSecret": {
-                    "description": "Secret of the service account used to access the Azure API.",
-                    "type": "string"
-                },
-                "resourceGroup": {
-                    "description": "Resource group in which the cluster is running.",
-                    "type": "string"
-                },
-                "subscriptionID": {
-                    "description": "Subscription ID of the Azure subscription.",
-                    "type": "string"
-                },
-                "tenantID": {
-                    "description": "Tenant ID of the Azure subscription.",
-                    "type": "string"
-                }
-            },
-            "required": [
-                "clientID",
-                "clientSecret",
-                "resourceGroup",
-                "subscriptionID",
-                "tenantID"
-            ]
         }
     },
     "required": [
         "csp",
         "image"
     ],
-    "if": {
-        "properties": {
-            "csp": {
-                "const": "Azure"
-            }
-        },
-        "required": [
-            "csp"
-        ]
-    },
-    "then": {
-        "required": [
-            "Azure"
-        ]
-    },
     "title": "Values",
     "type": "object"
 }

cli/internal/helm/testdata/Azure/constellation-services/charts/autoscaler/templates/azure-deployment.yaml

@@ -23,43 +23,25 @@ spec:
         - name: cluster-autoscaler
           image: autoscalerImage
           imagePullPolicy: IfNotPresent
+          command:
+          - ./cluster-autoscaler
+          args:
+          - --cloud-provider=azure
+          - --cloud-config=/etc/azure/azure.json
+          - --logtostderr=true
+          - --namespace=kube-system
+          - --stderrthreshold=info
+          - --v=2
+          volumeMounts:
+          - name: azureconfig
+            mountPath: /etc/azure
+            readOnly: true
           livenessProbe:
             httpGet:
               path: /health-check
               port: 8085
           ports:
             - containerPort: 8085
-          env:
-            - name: ARM_SUBSCRIPTION_ID
-              valueFrom:
-                secretKeyRef:
-                  key: SubscriptionID
-                  name: cluster-autoscaler-azure
-            - name: ARM_RESOURCE_GROUP
-              valueFrom:
-                secretKeyRef:
-                  key: ResourceGroup
-                  name: cluster-autoscaler-azure
-            - name: ARM_TENANT_ID
-              valueFrom:
-                secretKeyRef:
-                  key: TenantID
-                  name: cluster-autoscaler-azure
-            - name: ARM_CLIENT_ID
-              valueFrom:
-                secretKeyRef:
-                  key: ClientID
-                  name: cluster-autoscaler-azure
-            - name: ARM_CLIENT_SECRET
-              valueFrom:
-                secretKeyRef:
-                  key: ClientSecret
-                  name: cluster-autoscaler-azure
-            - name: ARM_VM_TYPE
-              valueFrom:
-                secretKeyRef:
-                  key: VMType
-                  name: cluster-autoscaler-azure
           resources: {}
       dnsPolicy: ClusterFirst
       nodeSelector:
@@ -77,3 +59,7 @@ spec:
           key: node.cloudprovider.kubernetes.io/uninitialized
           operator: Equal
           value: "true"
+      volumes:
+        - name: azureconfig
+          secret:
+            secretName: azureconfig

internal/cloud/azure/azure.go

@@ -125,19 +125,30 @@ func (c *Cloud) GetCCMConfig(ctx context.Context, providerID string, cloudServic
 		return nil, fmt.Errorf("could not dereference load balancer name")
 	}
 
+	var uamiClientID string
+	useManagedIdentityExtension := creds.PreferredAuthMethod == azureshared.AuthMethodUserAssignedIdentity
+	if useManagedIdentityExtension {
+		uamiClientID, err = c.getUAMIClientIDFromURI(ctx, providerID, creds.UamiResourceID)
+		if err != nil {
+			return nil, fmt.Errorf("retrieving user-assigned managed identity client ID: %w", err)
+		}
+	}
+
 	config := cloudConfig{
-		Cloud:               "AzurePublicCloud",
-		TenantID:            creds.TenantID,
-		SubscriptionID:      subscriptionID,
-		ResourceGroup:       resourceGroup,
-		LoadBalancerSku:     "standard",
-		SecurityGroupName:   securityGroupName,
-		LoadBalancerName:    *loadBalancer.Name,
-		UseInstanceMetadata: true,
-		VMType:              "vmss",
-		Location:            creds.Location,
-		AADClientID:         creds.AppClientID,
-		AADClientSecret:     creds.ClientSecretValue,
+		Cloud:                       "AzurePublicCloud",
+		TenantID:                    creds.TenantID,
+		SubscriptionID:              subscriptionID,
+		ResourceGroup:               resourceGroup,
+		LoadBalancerSku:             "standard",
+		SecurityGroupName:           securityGroupName,
+		LoadBalancerName:            *loadBalancer.Name,
+		UseInstanceMetadata:         true,
+		VMType:                      "vmss",
+		Location:                    creds.Location,
+		UseManagedIdentityExtension: useManagedIdentityExtension,
+		UserAssignedIdentityID:      uamiClientID,
+		AADClientID:                 creds.AppClientID,
+		AADClientSecret:             creds.ClientSecretValue,
 	}
 
 	return json.Marshal(config)
@@ -304,6 +315,24 @@ func (c *Cloud) getInstance(ctx context.Context, providerID string) (metadata.In
 	return instance, nil
 }
 
+func (c *Cloud) getUAMIClientIDFromURI(ctx context.Context, providerID, resourceID string) (string, error) {
+	// userAssignedIdentityURI := "/subscriptions/{subscription-id}/resourcegroups/{resource-group}/providers/Microsoft.ManagedIdentity/userAssignedIdentities/{identity-name}"
+	_, resourceGroup, scaleSet, instanceID, err := azureshared.ScaleSetInformationFromProviderID(providerID)
+	if err != nil {
+		return "", fmt.Errorf("invalid provider ID: %w", err)
+	}
+	vmResp, err := c.scaleSetsVMAPI.Get(ctx, resourceGroup, scaleSet, instanceID, nil)
+	if err != nil {
+		return "", fmt.Errorf("retrieving instance: %w", err)
+	}
+	for rID, v := range vmResp.Identity.UserAssignedIdentities {
+		if rID == resourceID {
+			return *v.ClientID, nil
+		}
+	}
+	return "", fmt.Errorf("no user assinged identity found for resource ID %s", resourceID)
+}
+
 // getNetworkSecurityGroupName returns the security group name of the resource group.
 func (c *Cloud) getNetworkSecurityGroupName(ctx context.Context, resourceGroup, uid string) (string, error) {
 	pager := c.secGroupAPI.NewListPager(resourceGroup, nil)
@@ -383,23 +412,25 @@ func (c *Cloud) getVMInterfaces(ctx context.Context, vm armcompute.VirtualMachin
 }
 
 type cloudConfig struct {
-	Cloud                      string `json:"cloud,omitempty"`
-	TenantID                   string `json:"tenantId,omitempty"`
-	SubscriptionID             string `json:"subscriptionId,omitempty"`
-	ResourceGroup              string `json:"resourceGroup,omitempty"`
-	Location                   string `json:"location,omitempty"`
-	SubnetName                 string `json:"subnetName,omitempty"`
-	SecurityGroupName          string `json:"securityGroupName,omitempty"`
-	SecurityGroupResourceGroup string `json:"securityGroupResourceGroup,omitempty"`
-	LoadBalancerName           string `json:"loadBalancerName,omitempty"`
-	LoadBalancerSku            string `json:"loadBalancerSku,omitempty"`
-	VNetName                   string `json:"vnetName,omitempty"`
-	VNetResourceGroup          string `json:"vnetResourceGroup,omitempty"`
-	CloudProviderBackoff       bool   `json:"cloudProviderBackoff,omitempty"`
-	UseInstanceMetadata        bool   `json:"useInstanceMetadata,omitempty"`
-	VMType                     string `json:"vmType,omitempty"`
-	AADClientID                string `json:"aadClientId,omitempty"`
-	AADClientSecret            string `json:"aadClientSecret,omitempty"`
+	Cloud                       string `json:"cloud,omitempty"`
+	TenantID                    string `json:"tenantId,omitempty"`
+	SubscriptionID              string `json:"subscriptionId,omitempty"`
+	ResourceGroup               string `json:"resourceGroup,omitempty"`
+	Location                    string `json:"location,omitempty"`
+	SubnetName                  string `json:"subnetName,omitempty"`
+	SecurityGroupName           string `json:"securityGroupName,omitempty"`
+	SecurityGroupResourceGroup  string `json:"securityGroupResourceGroup,omitempty"`
+	LoadBalancerName            string `json:"loadBalancerName,omitempty"`
+	LoadBalancerSku             string `json:"loadBalancerSku,omitempty"`
+	VNetName                    string `json:"vnetName,omitempty"`
+	VNetResourceGroup           string `json:"vnetResourceGroup,omitempty"`
+	CloudProviderBackoff        bool   `json:"cloudProviderBackoff,omitempty"`
+	UseInstanceMetadata         bool   `json:"useInstanceMetadata,omitempty"`
+	VMType                      string `json:"vmType,omitempty"`
+	UseManagedIdentityExtension bool   `json:"useManagedIdentityExtension,omitempty"`
+	UserAssignedIdentityID      string `json:"userAssignedIdentityID,omitempty"`
+	AADClientID                 string `json:"aadClientId,omitempty"`
+	AADClientSecret             string `json:"aadClientSecret,omitempty"`
 }
 
 // convertToInstanceMetadata converts a armcomputev2.VirtualMachineScaleSetVM to a metadata.InstanceMetadata.

internal/cloud/azure/azure_test.go

@@ -52,10 +52,13 @@ func TestGetCCMConfig(t *testing.T) {
 		Name: to.Ptr("security-group"),
 	}
 
+	uamiClientID := "uami-client-id"
+
 	testCases := map[string]struct {
 		imdsAPI                imdsAPI
 		loadBalancerAPI        loadBalancerAPI
 		secGroupAPI            securityGroupsAPI
+		scaleSetsVMAPI         virtualMachineScaleSetVMsAPI
 		providerID             string
 		cloudServiceAccountURI string
 		wantErr                bool
@@ -75,8 +78,50 @@ func TestGetCCMConfig(t *testing.T) {
 					list: []armnetwork.SecurityGroup{goodSecurityGroup},
 				},
 			},
+			scaleSetsVMAPI: &stubVirtualMachineScaleSetVMsAPI{
+				getVM: armcompute.VirtualMachineScaleSetVM{
+					Identity: &armcompute.VirtualMachineIdentity{
+						UserAssignedIdentities: map[string]*armcompute.UserAssignedIdentitiesValue{
+							"subscriptions/9b352db0-82af-408c-a02c-36fbffbf7015/resourceGroups/resourceGroupName/providers/Microsoft.ManagedIdentity/userAssignedIdentities/UAMIName": {ClientID: &uamiClientID},
+						},
+					},
+				},
+			},
 			providerID:             "azure:///subscriptions/subscription-id/resourceGroups/resource-group/providers/Microsoft.Compute/virtualMachineScaleSets/scale-set/virtualMachines/0",
-			cloudServiceAccountURI: "serviceaccount://azure?tenant_id=tenant-id&client_id=client-id&client_secret=client-secret&location=westeurope",
+			cloudServiceAccountURI: "serviceaccount://azure?tenant_id=tenant-id&client_id=client-id&client_secret=client-secret&location=westeurope&preferred_auth_method=userassignedidentity&uami_resource_id=subscriptions%2F9b352db0-82af-408c-a02c-36fbffbf7015%2FresourceGroups%2FresourceGroupName%2Fproviders%2FMicrosoft.ManagedIdentity%2FuserAssignedIdentities%2FUAMIName",
+			wantConfig: cloudConfig{
+				Cloud:                       "AzurePublicCloud",
+				TenantID:                    "tenant-id",
+				SubscriptionID:              "subscription-id",
+				ResourceGroup:               "resource-group",
+				LoadBalancerSku:             "standard",
+				SecurityGroupName:           "security-group",
+				LoadBalancerName:            "load-balancer",
+				UseInstanceMetadata:         true,
+				UseManagedIdentityExtension: true,
+				UserAssignedIdentityID:      uamiClientID,
+				VMType:                      "vmss",
+				Location:                    "westeurope",
+				AADClientID:                 "client-id",
+				AADClientSecret:             "client-secret",
+			},
+		},
+		"no app registration": {
+			imdsAPI: &stubIMDSAPI{
+				uidVal: "uid",
+			},
+			loadBalancerAPI: &stubLoadBalancersAPI{
+				pager: &stubLoadBalancersClientListPager{
+					list: []armnetwork.LoadBalancer{goodLB},
+				},
+			},
+			secGroupAPI: &stubSecurityGroupsAPI{
+				pager: &stubSecurityGroupsClientListPager{
+					list: []armnetwork.SecurityGroup{goodSecurityGroup},
+				},
+			},
+			providerID:             "azure:///subscriptions/subscription-id/resourceGroups/resource-group/providers/Microsoft.Compute/virtualMachineScaleSets/scale-set/virtualMachines/0",
+			cloudServiceAccountURI: "serviceaccount://azure?tenant_id=tenant-id&location=westeurope",
 			wantConfig: cloudConfig{
 				Cloud:               "AzurePublicCloud",
 				TenantID:            "tenant-id",
@@ -88,8 +133,6 @@ func TestGetCCMConfig(t *testing.T) {
 				UseInstanceMetadata: true,
 				VMType:              "vmss",
 				Location:            "westeurope",
-				AADClientID:         "client-id",
-				AADClientSecret:     "client-secret",
 			},
 		},
 		"missing UID tag": {
@@ -303,6 +346,7 @@ func TestGetCCMConfig(t *testing.T) {
 				imds:            tc.imdsAPI,
 				loadBalancerAPI: tc.loadBalancerAPI,
 				secGroupAPI:     tc.secGroupAPI,
+				scaleSetsVMAPI:  tc.scaleSetsVMAPI,
 			}
 			config, err := cloud.GetCCMConfig(context.Background(), tc.providerID, tc.cloudServiceAccountURI)
 			if tc.wantErr {

internal/cloud/azureshared/BUILD.bazel

@@ -5,6 +5,7 @@ go_library(
     name = "azureshared",
     srcs = [
         "appcredentials.go",
+        "authmethod_string.go",
         "azureshared.go",
         "metadata.go",
     ],

internal/cloud/azureshared/appcredentials.go

@@ -9,15 +9,19 @@ package azureshared
 import (
 	"fmt"
 	"net/url"
+	"strings"
 )
 
-// ApplicationCredentials is a set of Azure AD application credentials.
+// ApplicationCredentials is a set of Azure API credentials.
+// It can contain a client secret and carries the preferred authentication method.
 // It is the equivalent of a service account key in other cloud providers.
 type ApplicationCredentials struct {
-	TenantID          string
-	AppClientID       string
-	ClientSecretValue string
-	Location          string
+	TenantID            string
+	AppClientID         string
+	ClientSecretValue   string
+	Location            string
+	UamiResourceID      string
+	PreferredAuthMethod AuthMethod
 }
 
 // ApplicationCredentialsFromURI converts a cloudServiceAccountURI into Azure ApplicationCredentials.
@@ -33,11 +37,14 @@ func ApplicationCredentialsFromURI(cloudServiceAccountURI string) (ApplicationCr
 		return ApplicationCredentials{}, fmt.Errorf("invalid service account URI: invalid host: %s", uri.Host)
 	}
 	query := uri.Query()
+	preferredAuthMethod := FromString(query.Get("preferred_auth_method"))
 	return ApplicationCredentials{
-		TenantID:          query.Get("tenant_id"),
-		AppClientID:       query.Get("client_id"),
-		ClientSecretValue: query.Get("client_secret"),
-		Location:          query.Get("location"),
+		TenantID:            query.Get("tenant_id"),
+		AppClientID:         query.Get("client_id"),
+		ClientSecretValue:   query.Get("client_secret"),
+		Location:            query.Get("location"),
+		UamiResourceID:      query.Get("uami_resource_id"),
+		PreferredAuthMethod: preferredAuthMethod,
 	}, nil
 }
 
@@ -45,9 +52,19 @@ func ApplicationCredentialsFromURI(cloudServiceAccountURI string) (ApplicationCr
 func (c ApplicationCredentials) ToCloudServiceAccountURI() string {
 	query := url.Values{}
 	query.Add("tenant_id", c.TenantID)
-	query.Add("client_id", c.AppClientID)
-	query.Add("client_secret", c.ClientSecretValue)
 	query.Add("location", c.Location)
+	if c.AppClientID != "" {
+		query.Add("client_id", c.AppClientID)
+	}
+	if c.ClientSecretValue != "" {
+		query.Add("client_secret", c.ClientSecretValue)
+	}
+	if c.UamiResourceID != "" {
+		query.Add("uami_resource_id", c.UamiResourceID)
+	}
+	if c.PreferredAuthMethod != AuthMethodUnknown {
+		query.Add("preferred_auth_method", c.PreferredAuthMethod.String())
+	}
 	uri := url.URL{
 		Scheme:   "serviceaccount",
 		Host:     "azure",
@@ -55,3 +72,29 @@ func (c ApplicationCredentials) ToCloudServiceAccountURI() string {
 	}
 	return uri.String()
 }
+
+//go:generate stringer -type=AuthMethod -trimprefix=AuthMethod
+
+// AuthMethod is the authentication method used for the Azure API.
+type AuthMethod uint32
+
+// FromString converts a string into an AuthMethod.
+func FromString(s string) AuthMethod {
+	switch strings.ToLower(s) {
+	case strings.ToLower(AuthMethodServicePrincipal.String()):
+		return AuthMethodServicePrincipal
+	case strings.ToLower(AuthMethodUserAssignedIdentity.String()):
+		return AuthMethodUserAssignedIdentity
+	default:
+		return AuthMethodUnknown
+	}
+}
+
+const (
+	// AuthMethodUnknown is default value for AuthMethod.
+	AuthMethodUnknown AuthMethod = iota
+	// AuthMethodServicePrincipal uses a client ID and secret.
+	AuthMethodServicePrincipal
+	// AuthMethodUserAssignedIdentity uses a user assigned identity.
+	AuthMethodUserAssignedIdentity
+)

internal/cloud/azureshared/appcredentials_test.go

@@ -21,6 +21,20 @@ func TestMain(m *testing.M) {
 
 func TestApplicationCredentialsFromURI(t *testing.T) {
 	creds := ApplicationCredentials{
+		TenantID:            "tenant-id",
+		AppClientID:         "client-id",
+		ClientSecretValue:   "client-secret",
+		Location:            "location",
+		UamiResourceID:      "subscriptions/9b352db0-82af-408c-a02c-36fbffbf7015/resourceGroups/resourceGroupName/providers/Microsoft.ManagedIdentity/userAssignedIdentities/UAMIName",
+		PreferredAuthMethod: AuthMethodServicePrincipal,
+	}
+	credsWithoutSecret := ApplicationCredentials{
+		TenantID:            "tenant-id",
+		Location:            "location",
+		UamiResourceID:      "subscriptions/9b352db0-82af-408c-a02c-36fbffbf7015/resourceGroups/resourceGroupName/providers/Microsoft.ManagedIdentity/userAssignedIdentities/UAMIName",
+		PreferredAuthMethod: AuthMethodUserAssignedIdentity,
+	}
+	credsWithoutPreferrredAuthMethod := ApplicationCredentials{
 		TenantID:          "tenant-id",
 		AppClientID:       "client-id",
 		ClientSecretValue: "client-secret",
@@ -32,9 +46,17 @@ func TestApplicationCredentialsFromURI(t *testing.T) {
 		wantErr                bool
 	}{
 		"getApplicationCredentials works": {
-			cloudServiceAccountURI: "serviceaccount://azure?tenant_id=tenant-id&client_id=client-id&client_secret=client-secret&location=location",
+			cloudServiceAccountURI: "serviceaccount://azure?tenant_id=tenant-id&client_id=client-id&client_secret=client-secret&location=location&preferred_auth_method=serviceprincipal&uami_resource_id=subscriptions%2F9b352db0-82af-408c-a02c-36fbffbf7015%2FresourceGroups%2FresourceGroupName%2Fproviders%2FMicrosoft.ManagedIdentity%2FuserAssignedIdentities%2FUAMIName",
 			wantCreds:              creds,
 		},
+		"can parse URI without app registration / secret": {
+			cloudServiceAccountURI: "serviceaccount://azure?tenant_id=tenant-id&location=location&preferred_auth_method=userassignedidentity&uami_resource_id=subscriptions%2F9b352db0-82af-408c-a02c-36fbffbf7015%2FresourceGroups%2FresourceGroupName%2Fproviders%2FMicrosoft.ManagedIdentity%2FuserAssignedIdentities%2FUAMIName",
+			wantCreds:              credsWithoutSecret,
+		},
+		"can parse URI without preferred auth method": {
+			cloudServiceAccountURI: "serviceaccount://azure?tenant_id=tenant-id&client_id=client-id&client_secret=client-secret&location=location",
+			wantCreds:              credsWithoutPreferrredAuthMethod,
+		},
 		"invalid URI fails": {
 			cloudServiceAccountURI: "\x00",
 			wantErr:                true,
@@ -66,25 +88,66 @@ func TestApplicationCredentialsFromURI(t *testing.T) {
 }
 
 func TestToCloudServiceAccountURI(t *testing.T) {
-	assert := assert.New(t)
-	require := require.New(t)
-	key := ApplicationCredentials{
-		TenantID:          "tenant-id",
-		AppClientID:       "client-id",
-		ClientSecretValue: "client-secret",
-		Location:          "location",
+	testCases := map[string]struct {
+		credentials   ApplicationCredentials
+		wantURLValues url.Values
+	}{
+		"client id and secret without preferred auth method": {
+			credentials: ApplicationCredentials{
+				TenantID:          "tenant-id",
+				AppClientID:       "client-id",
+				ClientSecretValue: "client-secret",
+				Location:          "location",
+			},
+			wantURLValues: url.Values{
+				"tenant_id":     []string{"tenant-id"},
+				"client_id":     []string{"client-id"},
+				"client_secret": []string{"client-secret"},
+				"location":      []string{"location"},
+			},
+		},
+		"client id and secret with preferred auth method": {
+			credentials: ApplicationCredentials{
+				TenantID:            "tenant-id",
+				AppClientID:         "client-id",
+				ClientSecretValue:   "client-secret",
+				Location:            "location",
+				PreferredAuthMethod: AuthMethodServicePrincipal,
+			},
+			wantURLValues: url.Values{
+				"tenant_id":             []string{"tenant-id"},
+				"client_id":             []string{"client-id"},
+				"client_secret":         []string{"client-secret"},
+				"location":              []string{"location"},
+				"preferred_auth_method": []string{"ServicePrincipal"},
+			},
+		},
+		"only preferred auth method": {
+			credentials: ApplicationCredentials{
+				TenantID:            "tenant-id",
+				Location:            "location",
+				PreferredAuthMethod: AuthMethodUserAssignedIdentity,
+			},
+			wantURLValues: url.Values{
+				"tenant_id":             []string{"tenant-id"},
+				"location":              []string{"location"},
+				"preferred_auth_method": []string{"UserAssignedIdentity"},
+			},
+		},
 	}
 
-	cloudServiceAccountURI := key.ToCloudServiceAccountURI()
-	uri, err := url.Parse(cloudServiceAccountURI)
-	require.NoError(err)
-	query := uri.Query()
-	assert.Equal("serviceaccount", uri.Scheme)
-	assert.Equal("azure", uri.Host)
-	assert.Equal(url.Values{
-		"tenant_id":     []string{"tenant-id"},
-		"client_id":     []string{"client-id"},
-		"client_secret": []string{"client-secret"},
-		"location":      []string{"location"},
-	}, query)
+	for name, tc := range testCases {
+		t.Run(name, func(t *testing.T) {
+			assert := assert.New(t)
+			require := require.New(t)
+
+			cloudServiceAccountURI := tc.credentials.ToCloudServiceAccountURI()
+			uri, err := url.Parse(cloudServiceAccountURI)
+			require.NoError(err)
+			query := uri.Query()
+			assert.Equal("serviceaccount", uri.Scheme)
+			assert.Equal("azure", uri.Host)
+			assert.Equal(tc.wantURLValues, query)
+		})
+	}
 }

internal/cloud/azureshared/authmethod_string.go

@@ -0,0 +1,25 @@
+// Code generated by "stringer -type=AuthMethod -trimprefix=AuthMethod"; DO NOT EDIT.
+
+package azureshared
+
+import "strconv"
+
+func _() {
+	// An "invalid array index" compiler error signifies that the constant values have changed.
+	// Re-run the stringer command to generate them again.
+	var x [1]struct{}
+	_ = x[AuthMethodUnknown-0]
+	_ = x[AuthMethodServicePrincipal-1]
+	_ = x[AuthMethodUserAssignedIdentity-2]
+}
+
+const _AuthMethod_name = "UnknownServicePrincipalUserAssignedIdentity"
+
+var _AuthMethod_index = [...]uint8{0, 7, 23, 43}
+
+func (i AuthMethod) String() string {
+	if i >= AuthMethod(len(_AuthMethod_index)-1) {
+		return "AuthMethod(" + strconv.FormatInt(int64(i), 10) + ")"
+	}
+	return _AuthMethod_name[_AuthMethod_index[i]:_AuthMethod_index[i+1]]
+}