[v16] Verify subscription id matches

lib/auth/join_azure.go

@@ -289,6 +289,9 @@ func verifyVMIdentity(
 	// from the VM resource.
 	vmSubscription, vmResourceGroup, err := claimsToIdentifiers(tokenClaims)
 	if err == nil {
+		if subscriptionID != vmSubscription {
+			return nil, trace.AccessDenied("subscription ID mismatch between attested data and access token")
+		}
 		return azureJoinToAttrs(vmSubscription, vmResourceGroup), nil
 	}
 	log.WithError(err).Warn("Failed to parse VM identifiers from claims. Retrying with Azure VM API.")

lib/auth/join_azure_test.go

@@ -754,6 +754,28 @@ func TestAuth_RegisterUsingAzureClaims(t *testing.T) {
 			certs:       []*x509.Certificate{tlsConfig.Certificate},
 			assertError: require.NoError,
 		},
+		{
+			name:                           "subscription mismatch between attestation and token",
+			requestTokenName:               "test-token",
+			tokenSubscription:              "attested-subscription",
+			tokenVMID:                      defaultVMID,
+			tokenManagedIdentityResourceID: vmResourceID("token-subscription", defaultResourceGroup, defaultVMName),
+			tokenSpec: types.ProvisionTokenSpecV2{
+				Roles: []types.SystemRole{types.RoleNode},
+				Azure: &types.ProvisionTokenSpecV2Azure{
+					Allow: []*types.ProvisionTokenSpecV2Azure_Rule{
+						{
+							Subscription:   "token-subscription",
+							ResourceGroups: []string{defaultResourceGroup},
+						},
+					},
+				},
+				JoinMethod: types.JoinMethodAzure,
+			},
+			verify:      mockVerifyToken(nil),
+			certs:       []*x509.Certificate{tlsConfig.Certificate},
+			assertError: isAccessDenied,
+		},
 	}
 
 	for _, tc := range tests {