[codex] Harden C# expression host-code execution (#7519)

* Harden C# expression host-code execution

* Address script authorization review feedback

* Harden script authorization failure responses

* Address code quality review feedback

* Use explicit failure filter in script authorization

* Centralize script activity type names

* Address script authorization review feedback

doc/wiki/expressions-and-scripting.md

@@ -55,11 +55,14 @@ Additional JavaScript libraries are in [Elsa.Expressions.JavaScript.Libraries](.
 ```csharp
 elsa.UseCSharp(options =>
 {
+    options.AllowHostCodeExecution = true;
     options.DisableWrappers = disableVariableWrappers;
     options.AppendScript("string Greet(string name) => $\"Hello {name}!\";");
 });
 ```
 
+Roslyn C# scripting is privileged host-code execution, not a sandbox. Hosts must explicitly set `CSharpOptions.AllowHostCodeExecution` to `true` before C# expressions or `RunCSharp` can be authored or executed. API callers that author, publish, dispatch, or directly execute workflows containing C# must have the `exec:csharp-expressions` permission.
+
 ## Python
 
 [PythonFeature](../../src/modules/Elsa.Expressions.Python/Features/PythonFeature.cs) registers pythonnet-based evaluation and configures `PythonGlobalInterpreterManager` as a hosted service. Python.NET execution is privileged host-code execution, not a sandbox. Python code can access host process capabilities through pythonnet and must only be enabled for trusted workflow authors.

src/apps/Elsa.Server.Web/Program.cs

@@ -86,6 +86,7 @@ services
             .UseScheduling()
             .UseCSharp(options =>
             {
+                configuration.GetSection("Scripting:CSharp").Bind(options);
                 options.DisableWrappers = disableVariableWrappers;
                 options.AppendScript("string Greet(string name) => $\"Hello {name}!\";");
                 options.AppendScript("string SayHelloWorld() => Greet(\"World\");");

src/apps/Elsa.Server.Web/appsettings.json

@@ -84,6 +84,9 @@
     ]
   },
   "Scripting": {
+    "CSharp": {
+      "AllowHostCodeExecution": false
+    },
     "Python": {
       "AllowHostCodeExecution": true,
       "PythonDllPath": "",

src/common/Elsa.Api.Common/PermissionNames.cs

@@ -5,6 +5,11 @@ public static class PermissionNames
     public const string All = "*";
     public const string ClaimType = "permissions";
 
+    /// <summary>
+    /// Permission required to author or execute C# workflow expressions.
+    /// </summary>
+    public const string ExecuteCSharpExpressions = "exec:csharp-expressions";
+
     /// <summary>
     /// Permission required to author or execute Python.NET workflow expressions.
     /// </summary>

src/common/Elsa.Testing.Shared.Integration/TestApplicationBuilder.cs

@@ -36,7 +36,7 @@ public class TestApplicationBuilder
         _configureElsa += elsa => elsa
             .AddActivitiesFrom<WriteLine>()
             .UseScheduling()
-            .UseCSharp()
+            .UseCSharp(options => options.AllowHostCodeExecution = true)
             .UseJavaScript()
             .UseLiquid()
             .UseWorkflowManagement()
@@ -119,4 +119,4 @@ public class TestApplicationBuilder
         _configureElsa += elsa => elsa.UseFluentStorageProvider(storage => storage.BlobStorage = sp => StorageFactory.Blobs.DirectoryFiles(Path.Combine(workflowsDirectory)));
         return this;
     }
-}
+}

src/modules/Elsa.Expressions.CSharp/Activities/RunCSharp/RunCSharp.cs

@@ -15,7 +15,7 @@ namespace Elsa.Expressions.CSharp.Activities;
 /// <summary>
 /// Executes C# code.
 /// </summary>
-[Activity("Elsa", "Scripting", "Executes C# code", DisplayName = "Run C#")]
+[Activity(WorkflowScriptActivityTypeNames.Namespace, WorkflowScriptActivityTypeNames.RunCSharpType, 1, "Executes C# code", "Scripting", DisplayName = "Run C#")]
 public class RunCSharp : CodeActivity<object?>
 {
     /// <inheritdoc />
@@ -72,4 +72,4 @@ public class RunCSharp : CodeActivity<object?>
         // Complete the activity with the outcome.
         await context.CompleteActivityWithOutcomesAsync(outcomes);
     }
-}
+}

src/modules/Elsa.Expressions.CSharp/ActivityDescriptorModifiers/CSharpActivityDescriptorModifier.cs

@@ -0,0 +1,17 @@
+using Elsa.Expressions.CSharp.Options;
+using Elsa.Workflows;
+using Elsa.Workflows.Models;
+using Microsoft.Extensions.Options;
+
+namespace Elsa.Expressions.CSharp.ActivityDescriptorModifiers;
+
+internal class CSharpActivityDescriptorModifier(IOptions<CSharpOptions> options) : IActivityDescriptorModifier
+{
+    public void Modify(ActivityDescriptor descriptor)
+    {
+        if (descriptor.TypeName != WorkflowScriptActivityTypeNames.RunCSharp)
+            return;
+
+        descriptor.IsBrowsable = options.Value.AllowHostCodeExecution;
+    }
+}

src/modules/Elsa.Expressions.CSharp/Features/CSharpFeature.cs

@@ -1,5 +1,6 @@
 using Elsa.Caching.Features;
 using Elsa.Common.Features;
+using Elsa.Expressions.CSharp.ActivityDescriptorModifiers;
 using Elsa.Expressions.CSharp.Activities;
 using Elsa.Expressions.CSharp.Contracts;
 using Elsa.Expressions.CSharp.Options;
@@ -42,6 +43,7 @@ public class CSharpFeature : FeatureBase
         Services
             .AddExpressionDescriptorProvider<CSharpExpressionDescriptorProvider>()
             .AddScoped<ICSharpEvaluator, CSharpEvaluator>()
+            .AddSingleton<IActivityDescriptorModifier, CSharpActivityDescriptorModifier>()
             ;
 
         // Handlers.
@@ -53,4 +55,4 @@ public class CSharpFeature : FeatureBase
         // UI property handlers.
         Services.AddScoped<IPropertyUIHandler, RunCSharpOptionsProvider>();
     }
-}
+}

src/modules/Elsa.Expressions.CSharp/Options/CSharpOptions.cs

@@ -16,6 +16,12 @@ namespace Elsa.Expressions.CSharp.Options;
 /// </summary>
 public class CSharpOptions
 {
+    /// <summary>
+    /// Gets or sets whether workflow-authored Roslyn C# host-code execution is allowed.
+    /// Roslyn scripting is not a sandbox and can access host process capabilities.
+    /// </summary>
+    public bool AllowHostCodeExecution { get; set; }
+
     /// <summary>
     /// A list of callbacks that is invoked when a C# expression is evaluated. Use this to configure the <see cref="ScriptOptions"/>.
     /// </summary>
@@ -101,4 +107,4 @@ public class CSharpOptions
         ConfigureScriptCallbacks.Add((s, c) => s.ContinueWith(script(c)));
         return this;
     }
-}
+}

src/modules/Elsa.Expressions.CSharp/Providers/CSharpExpressionDescriptorProvider.cs

@@ -1,12 +1,14 @@
 using Elsa.Expressions.CSharp.Expressions;
+using Elsa.Expressions.CSharp.Options;
 using Elsa.Expressions.Contracts;
 using Elsa.Expressions.Models;
 using Elsa.Extensions;
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Options;
 
 namespace Elsa.Expressions.CSharp.Providers;
 
-internal class CSharpExpressionDescriptorProvider : IExpressionDescriptorProvider
+internal class CSharpExpressionDescriptorProvider(IOptions<CSharpOptions> options) : IExpressionDescriptorProvider
 {
     private const string TypeName = "CSharp";
 
@@ -16,8 +18,9 @@ internal class CSharpExpressionDescriptorProvider : IExpressionDescriptorProvide
         {
             Type = TypeName,
             DisplayName = "C#",
+            IsBrowsable = options.Value.AllowHostCodeExecution,
             Properties = new { MonacoLanguage = "csharp" }.ToDictionary(),
             HandlerFactory = ActivatorUtilities.GetServiceOrCreateInstance<CSharpExpressionHandler>
         };
     }
-}
+}

src/modules/Elsa.Expressions.CSharp/Services/CSharpEvaluator.cs

@@ -34,6 +34,9 @@ public class CSharpEvaluator(INotificationSender notificationSender, IOptions<CS
         Func<Script<object>, Script<object>>? configureScript = default,
         CancellationToken cancellationToken = default)
     {
+        if (!_csharpOptions.AllowHostCodeExecution)
+            throw new InvalidOperationException("C# workflow expression execution is disabled. Set CSharpOptions.AllowHostCodeExecution to true only for trusted workflow authors; Roslyn scripting is not a sandbox.");
+
         var scriptOptions = ScriptOptions.Default.WithOptimizationLevel(OptimizationLevel.Release);
 
         if (configureScriptOptions != null)
@@ -85,4 +88,4 @@ public class CSharpEvaluator(INotificationSender notificationSender, IOptions<CS
         var hash = SHA256.HashData(segment.AsSpan());
         return Convert.ToBase64String(hash);
     }
-}
+}

src/modules/Elsa.Expressions.CSharp/ShellFeatures/CSharpFeature.cs

@@ -1,4 +1,5 @@
 using CShells.Features;
+using Elsa.Expressions.CSharp.ActivityDescriptorModifiers;
 using Elsa.Expressions.CSharp.Activities;
 using Elsa.Expressions.CSharp.Contracts;
 using Elsa.Expressions.CSharp.Options;
@@ -33,7 +34,8 @@ public class CSharpFeature : IShellFeature
         // C# services.
         services
             .AddExpressionDescriptorProvider<CSharpExpressionDescriptorProvider>()
-            .AddScoped<ICSharpEvaluator, CSharpEvaluator>();
+            .AddScoped<ICSharpEvaluator, CSharpEvaluator>()
+            .AddSingleton<IActivityDescriptorModifier, CSharpActivityDescriptorModifier>();
 
         // Handlers.
         services.AddNotificationHandlersFrom<CSharpFeature>();
@@ -43,4 +45,3 @@ public class CSharpFeature : IShellFeature
     }
 }
 
-

src/modules/Elsa.Expressions.Python/Activities/RunPython/RunPython.cs

@@ -13,7 +13,7 @@ namespace Elsa.Expressions.Python.Activities;
 /// <summary>
 /// Executes Python code.
 /// </summary>
-[Activity("Elsa", "Scripting", "Executes Python code", DisplayName = "Run Python")]
+[Activity(WorkflowScriptActivityTypeNames.Namespace, WorkflowScriptActivityTypeNames.RunPythonType, 1, "Executes Python code", "Scripting", DisplayName = "Run Python")]
 public class RunPython : CodeActivity<object?>
 {
     /// <inheritdoc />
@@ -69,4 +69,4 @@ public class RunPython : CodeActivity<object?>
         // Complete the activity with the outcome.
         await context.CompleteActivityWithOutcomesAsync(outcomes);
     }
-}
+}

src/modules/Elsa.Expressions.Python/ActivityDescriptorModifiers/PythonActivityDescriptorModifier.cs

@@ -1,6 +1,5 @@
 using Elsa.Expressions.Python.Options;
 using Elsa.Workflows;
-using Elsa.Workflows.Helpers;
 using Elsa.Workflows.Models;
 using Microsoft.Extensions.Options;
 
@@ -8,11 +7,9 @@ namespace Elsa.Expressions.Python.ActivityDescriptorModifiers;
 
 internal class PythonActivityDescriptorModifier(IOptions<PythonOptions> options) : IActivityDescriptorModifier
 {
-    private static readonly string RunPythonActivityType = ActivityTypeNameHelper.GenerateTypeName<Activities.RunPython>();
-
     public void Modify(ActivityDescriptor descriptor)
     {
-        if (descriptor.TypeName != RunPythonActivityType)
+        if (descriptor.TypeName != WorkflowScriptActivityTypeNames.RunPython)
             return;
 
         descriptor.IsBrowsable = options.Value.AllowHostCodeExecution;

src/modules/Elsa.Workflows.Api/Endpoints/Scripting/ExpressionDescriptors/List/Endpoint.cs

@@ -1,3 +1,4 @@
+using System.Collections.Frozen;
 using Elsa.Abstractions;
 using Elsa.Expressions.Contracts;
 using Elsa.Expressions.Models;
@@ -12,7 +13,11 @@ namespace Elsa.Workflows.Api.Endpoints.Scripting.ExpressionDescriptors.List;
 [UsedImplicitly]
 internal class List(IExpressionDescriptorRegistry expressionDescriptorRegistry) : ElsaEndpointWithoutRequest<ListResponse<ExpressionDescriptorModel>>
 {
-    private const string PythonExpressionType = "Python";
+    private static readonly IReadOnlyDictionary<string, string> PrivilegedExpressionPermissions = new Dictionary<string, string>
+    {
+        ["CSharp"] = PermissionNames.ExecuteCSharpExpressions,
+        ["Python"] = PermissionNames.ExecutePythonExpressions
+    }.ToFrozenDictionary(StringComparer.Ordinal);
 
     /// <inheritdoc />
     public override void Configure()
@@ -32,9 +37,10 @@ internal class List(IExpressionDescriptorRegistry expressionDescriptorRegistry)
 
     private bool CanListDescriptor(ExpressionDescriptor descriptor)
     {
-        return descriptor.Type != PythonExpressionType ||
-               (descriptor.IsBrowsable &&
-                User.Claims.Any(x => x.Type == "permissions" && (x.Value == PermissionNames.All || x.Value == PermissionNames.ExecutePythonExpressions)));
+        if (!PrivilegedExpressionPermissions.TryGetValue(descriptor.Type, out var permission))
+            return true;
+
+        return descriptor.IsBrowsable && User.Claims.Any(x => x.Type == PermissionNames.ClaimType && (x.Value == PermissionNames.All || x.Value == permission));
     }
 
     private static IEnumerable<ExpressionDescriptorModel> Map(List<ExpressionDescriptor> descriptors) => descriptors.Select(Map);

src/modules/Elsa.Workflows.Api/Endpoints/WorkflowDefinitions/BulkDispatch/Endpoint.cs

@@ -14,7 +14,7 @@ internal class Endpoint(
     IWorkflowDefinitionService workflowDefinitionService,
     IWorkflowDispatcher workflowDispatcher,
     IIdentityGenerator identityGenerator,
-    PythonWorkflowDefinitionAuthorizationService pythonAuthorizationService)
+    WorkflowDefinitionScriptAuthorizationService scriptAuthorizationService)
     : ElsaEndpoint<Request, Response>
 {
     public override void Configure()
@@ -35,10 +35,10 @@ internal class Endpoint(
             return;
         }
 
-        var pythonAuthorizationResult = await pythonAuthorizationService.AuthorizeAsync(workflowGraph.Workflow, User, cancellationToken);
-        if (pythonAuthorizationResult != PythonWorkflowDefinitionAuthorizationResult.Allowed)
+        var scriptAuthorizationResult = await scriptAuthorizationService.AuthorizeAsync(workflowGraph.Workflow, User, cancellationToken);
+        if (!scriptAuthorizationResult.Succeeded)
         {
-            await PythonWorkflowDefinitionAuthorizationFailure.SendAsync(pythonAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
+            await WorkflowDefinitionScriptAuthorizationFailure.SendAsync(scriptAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
             return;
         }
 

src/modules/Elsa.Workflows.Api/Endpoints/WorkflowDefinitions/BulkPublish/Endpoint.cs

@@ -17,7 +17,7 @@ internal class BulkPublish(
     IWorkflowDefinitionPublisher workflowDefinitionPublisher,
     IAuthorizationService authorizationService,
     IWorkflowDefinitionService workflowDefinitionService,
-    PythonWorkflowDefinitionAuthorizationService pythonAuthorizationService)
+    WorkflowDefinitionScriptAuthorizationService scriptAuthorizationService)
     : ElsaEndpoint<Request, Response>
 {
     public override void Configure()
@@ -77,10 +77,10 @@ internal class BulkPublish(
         foreach (var (_, definition) in publishableDefinitions)
         {
             var workflowGraph = await workflowDefinitionService.MaterializeWorkflowAsync(definition, cancellationToken);
-            var pythonAuthorizationResult = await pythonAuthorizationService.AuthorizeAsync(workflowGraph.Workflow, User, cancellationToken);
-            if (pythonAuthorizationResult != PythonWorkflowDefinitionAuthorizationResult.Allowed)
+            var scriptAuthorizationResult = await scriptAuthorizationService.AuthorizeAsync(workflowGraph.Workflow, User, cancellationToken);
+            if (!scriptAuthorizationResult.Succeeded)
             {
-                await PythonWorkflowDefinitionAuthorizationFailure.SendAsync(pythonAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
+                await WorkflowDefinitionScriptAuthorizationFailure.SendAsync(scriptAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
                 return null!;
             }
         }

src/modules/Elsa.Workflows.Api/Endpoints/WorkflowDefinitions/Dispatch/Endpoint.cs

@@ -13,7 +13,7 @@ internal class Endpoint(
     IWorkflowDefinitionService workflowDefinitionService,
     IWorkflowDispatcher workflowDispatcher,
     IIdentityGenerator identityGenerator,
-    PythonWorkflowDefinitionAuthorizationService pythonAuthorizationService) : ElsaEndpoint<Request, Response>
+    WorkflowDefinitionScriptAuthorizationService scriptAuthorizationService) : ElsaEndpoint<Request, Response>
 {
     public override void Configure()
     {
@@ -33,10 +33,10 @@ internal class Endpoint(
             return;
         }
 
-        var pythonAuthorizationResult = await pythonAuthorizationService.AuthorizeAsync(workflowGraph.Workflow, User, cancellationToken);
-        if (pythonAuthorizationResult != PythonWorkflowDefinitionAuthorizationResult.Allowed)
+        var scriptAuthorizationResult = await scriptAuthorizationService.AuthorizeAsync(workflowGraph.Workflow, User, cancellationToken);
+        if (!scriptAuthorizationResult.Succeeded)
         {
-            await PythonWorkflowDefinitionAuthorizationFailure.SendAsync(pythonAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
+            await WorkflowDefinitionScriptAuthorizationFailure.SendAsync(scriptAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
             return;
         }
         

src/modules/Elsa.Workflows.Api/Endpoints/WorkflowDefinitions/Execute/GetEndpoint.cs

@@ -15,7 +15,7 @@ internal class GetEndpoint(
     IWorkflowRuntime workflowRuntime,
     IWorkflowStarter workflowStarter,
     IApiSerializer apiSerializer,
-    PythonWorkflowDefinitionAuthorizationService pythonAuthorizationService)
+    WorkflowDefinitionScriptAuthorizationService scriptAuthorizationService)
     : ElsaEndpoint<GetRequest>
 {
     /// <inheritdoc />
@@ -35,7 +35,7 @@ internal class GetEndpoint(
             workflowRuntime,
             workflowStarter,
             apiSerializer,
-            pythonAuthorizationService,
+            scriptAuthorizationService,
             HttpContext,
             cancellationToken);
     }

src/modules/Elsa.Workflows.Api/Endpoints/WorkflowDefinitions/Execute/PostEndpoint.cs

@@ -16,7 +16,7 @@ internal class PostEndpoint(
     IWorkflowRuntime workflowRuntime,
     IWorkflowStarter workflowStarter,
     IApiSerializer apiSerializer,
-    PythonWorkflowDefinitionAuthorizationService pythonAuthorizationService)
+    WorkflowDefinitionScriptAuthorizationService scriptAuthorizationService)
     : ElsaEndpointWithoutRequest<Response>
 {
     /// <inheritdoc />
@@ -73,7 +73,7 @@ internal class PostEndpoint(
             workflowRuntime,
             workflowStarter,
             apiSerializer,
-            pythonAuthorizationService,
+            scriptAuthorizationService,
             HttpContext,
             cancellationToken);
     }

src/modules/Elsa.Workflows.Api/Endpoints/WorkflowDefinitions/Execute/WorkflowExecutionHelper.cs

@@ -17,7 +17,7 @@ internal static class WorkflowExecutionHelper
         IWorkflowRuntime workflowRuntime,
         IWorkflowStarter workflowStarter,
         IApiSerializer apiSerializer,
-        PythonWorkflowDefinitionAuthorizationService pythonAuthorizationService,
+        WorkflowDefinitionScriptAuthorizationService scriptAuthorizationService,
         HttpContext httpContext,
         CancellationToken cancellationToken)
     {
@@ -31,10 +31,10 @@ internal static class WorkflowExecutionHelper
             return;
         }
 
-        var pythonAuthorizationResult = await pythonAuthorizationService.AuthorizeAsync(workflowGraph.Workflow, httpContext.User, cancellationToken);
-        if (pythonAuthorizationResult != PythonWorkflowDefinitionAuthorizationResult.Allowed)
+        var scriptAuthorizationResult = await scriptAuthorizationService.AuthorizeAsync(workflowGraph.Workflow, httpContext.User, cancellationToken);
+        if (!scriptAuthorizationResult.Succeeded)
         {
-            await SendPythonAuthorizationFailureAsync(httpContext, pythonAuthorizationResult, cancellationToken);
+            await SendScriptAuthorizationFailureAsync(httpContext, scriptAuthorizationResult, cancellationToken);
             return;
         }
         
@@ -97,15 +97,15 @@ internal static class WorkflowExecutionHelper
         await httpContext.Response.WriteAsync(faultedResponse, cancellationToken);
     }
 
-    private static async Task SendPythonAuthorizationFailureAsync(HttpContext httpContext, PythonWorkflowDefinitionAuthorizationResult result, CancellationToken cancellationToken)
+    private static async Task SendScriptAuthorizationFailureAsync(HttpContext httpContext, WorkflowDefinitionScriptAuthorizationResult result, CancellationToken cancellationToken)
     {
-        if (result == PythonWorkflowDefinitionAuthorizationResult.MissingPermission)
+        if (result.FailureReason == WorkflowDefinitionScriptAuthorizationFailureReason.MissingPermission)
         {
             await httpContext.Response.SendForbiddenAsync(cancellation: cancellationToken);
             return;
         }
 
         httpContext.Response.StatusCode = StatusCodes.Status400BadRequest;
-        await httpContext.Response.WriteAsync(PythonWorkflowDefinitionAuthorizationService.HostDisabledMessage, cancellationToken);
+        await httpContext.Response.WriteAsync(result.Message ?? "Workflow script authorization failed.", cancellationToken);
     }
 }

src/modules/Elsa.Workflows.Api/Endpoints/WorkflowDefinitions/Import/Endpoint.cs

@@ -17,7 +17,7 @@ internal class Import : ElsaEndpoint<WorkflowDefinitionModel>
     private readonly IWorkflowDefinitionImporter _workflowDefinitionImporter;
     private readonly IWorkflowDefinitionLinker _linker;
     private readonly IAuthorizationService _authorizationService;
-    private readonly PythonWorkflowDefinitionAuthorizationService _pythonAuthorizationService;
+    private readonly WorkflowDefinitionScriptAuthorizationService _scriptAuthorizationService;
 
     /// <inheritdoc />
     public Import(
@@ -25,13 +25,13 @@ internal class Import : ElsaEndpoint<WorkflowDefinitionModel>
         IWorkflowDefinitionImporter workflowDefinitionImporter,
         IWorkflowDefinitionLinker linker,
         IAuthorizationService authorizationService,
-        PythonWorkflowDefinitionAuthorizationService pythonAuthorizationService)
+        WorkflowDefinitionScriptAuthorizationService scriptAuthorizationService)
     {
         _workflowDefinitionStore = workflowDefinitionStore;
         _workflowDefinitionImporter = workflowDefinitionImporter;
         _linker = linker;
         _authorizationService = authorizationService;
-        _pythonAuthorizationService = pythonAuthorizationService;
+        _scriptAuthorizationService = scriptAuthorizationService;
     }
 
     /// <inheritdoc />
@@ -48,10 +48,10 @@ internal class Import : ElsaEndpoint<WorkflowDefinitionModel>
         var definitionId = model.DefinitionId;
         var isNew = string.IsNullOrWhiteSpace(definitionId);
 
-        var pythonAuthorizationResult = await _pythonAuthorizationService.AuthorizeAsync(model, User, cancellationToken);
-        if (pythonAuthorizationResult != PythonWorkflowDefinitionAuthorizationResult.Allowed)
+        var scriptAuthorizationResult = await _scriptAuthorizationService.AuthorizeAsync(model, User, cancellationToken);
+        if (!scriptAuthorizationResult.Succeeded)
         {
-            await PythonWorkflowDefinitionAuthorizationFailure.SendAsync(pythonAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
+            await WorkflowDefinitionScriptAuthorizationFailure.SendAsync(scriptAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
             return;
         }
 

src/modules/Elsa.Workflows.Api/Endpoints/WorkflowDefinitions/ImportFiles/Endpoint.cs

@@ -18,7 +18,7 @@ internal class ImportFiles : ElsaEndpoint<WorkflowDefinitionModel>
     private readonly IWorkflowDefinitionImporter _workflowDefinitionImporter;
     private readonly IApiSerializer _apiSerializer;
     private readonly IAuthorizationService _authorizationService;
-    private readonly PythonWorkflowDefinitionAuthorizationService _pythonAuthorizationService;
+    private readonly WorkflowDefinitionScriptAuthorizationService _scriptAuthorizationService;
 
     /// <inheritdoc />
     public ImportFiles(
@@ -26,13 +26,13 @@ internal class ImportFiles : ElsaEndpoint<WorkflowDefinitionModel>
         IWorkflowDefinitionImporter workflowDefinitionImporter,
         IApiSerializer apiSerializer,
         IAuthorizationService authorizationService,
-        PythonWorkflowDefinitionAuthorizationService pythonAuthorizationService)
+        WorkflowDefinitionScriptAuthorizationService scriptAuthorizationService)
     {
         _workflowDefinitionStore = workflowDefinitionStore;
         _workflowDefinitionImporter = workflowDefinitionImporter;
         _apiSerializer = apiSerializer;
         _authorizationService = authorizationService;
-        _pythonAuthorizationService = pythonAuthorizationService;
+        _scriptAuthorizationService = scriptAuthorizationService;
     }
 
     /// <inheritdoc />
@@ -80,10 +80,10 @@ internal class ImportFiles : ElsaEndpoint<WorkflowDefinitionModel>
     {
         foreach (var model in models)
         {
-            var pythonAuthorizationResult = await _pythonAuthorizationService.AuthorizeAsync(model, User, cancellationToken);
-            if (pythonAuthorizationResult != PythonWorkflowDefinitionAuthorizationResult.Allowed)
+            var scriptAuthorizationResult = await _scriptAuthorizationService.AuthorizeAsync(model, User, cancellationToken);
+            if (!scriptAuthorizationResult.Succeeded)
             {
-                await PythonWorkflowDefinitionAuthorizationFailure.SendAsync(pythonAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
+                await WorkflowDefinitionScriptAuthorizationFailure.SendAsync(scriptAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
                 return false;
             }
         }

src/modules/Elsa.Workflows.Api/Endpoints/WorkflowDefinitions/Post/Endpoint.cs

@@ -26,7 +26,7 @@ internal class Post(
     IDistributedLockProvider distributedLockProvider,
     IWorkflowDefinitionLinker linker,
     IAuthorizationService authorizationService,
-    PythonWorkflowDefinitionAuthorizationService pythonAuthorizationService)
+    WorkflowDefinitionScriptAuthorizationService scriptAuthorizationService)
     : ElsaEndpoint<SaveWorkflowDefinitionRequest, LinkedWorkflowDefinitionModel>
 {
     public override void Configure()
@@ -66,10 +66,10 @@ internal class Post(
             return;
         }
 
-        var pythonAuthorizationResult = await pythonAuthorizationService.AuthorizeAsync(model, User, cancellationToken);
-        if (pythonAuthorizationResult != PythonWorkflowDefinitionAuthorizationResult.Allowed)
+        var scriptAuthorizationResult = await scriptAuthorizationService.AuthorizeAsync(model, User, cancellationToken);
+        if (!scriptAuthorizationResult.Succeeded)
         {
-            await PythonWorkflowDefinitionAuthorizationFailure.SendAsync(pythonAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
+            await WorkflowDefinitionScriptAuthorizationFailure.SendAsync(scriptAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
             return;
         }
 

src/modules/Elsa.Workflows.Api/Endpoints/WorkflowDefinitions/Publish/Endpoint.cs

@@ -17,7 +17,7 @@ internal class Publish(
     IWorkflowDefinitionLinker linker,
     IAuthorizationService authorizationService,
     IWorkflowDefinitionService workflowDefinitionService,
-    PythonWorkflowDefinitionAuthorizationService pythonAuthorizationService)
+    WorkflowDefinitionScriptAuthorizationService scriptAuthorizationService)
     : ElsaEndpoint<Request, Response>
 {
     public override void Configure()
@@ -51,10 +51,10 @@ internal class Publish(
         }
 
         var workflowGraph = await workflowDefinitionService.MaterializeWorkflowAsync(definition, cancellationToken);
-        var pythonAuthorizationResult = await pythonAuthorizationService.AuthorizeAsync(workflowGraph.Workflow, User, cancellationToken);
-        if (pythonAuthorizationResult != PythonWorkflowDefinitionAuthorizationResult.Allowed)
+        var scriptAuthorizationResult = await scriptAuthorizationService.AuthorizeAsync(workflowGraph.Workflow, User, cancellationToken);
+        if (!scriptAuthorizationResult.Succeeded)
         {
-            await PythonWorkflowDefinitionAuthorizationFailure.SendAsync(pythonAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
+            await WorkflowDefinitionScriptAuthorizationFailure.SendAsync(scriptAuthorizationResult, Send.ForbiddenAsync, message => AddError(message), Send.ErrorsAsync, cancellationToken);
             return;
         }
 

src/modules/Elsa.Workflows.Api/Features/WorkflowsApiFeature.cs

@@ -37,7 +37,7 @@ public class WorkflowsApiFeature(IModule module) : FeatureBase(module)
         Module.AddFastEndpointsFromModule();
 
         Services.AddScoped<IWorkflowDefinitionLinker, StaticWorkflowDefinitionLinker>();
-        Services.AddScoped<PythonWorkflowDefinitionAuthorizationService>();
+        Services.AddScoped<WorkflowDefinitionScriptAuthorizationService>();
         Services.AddScoped<IAuthorizationHandler, NotReadOnlyRequirementHandler>();
         Services.Configure<AuthorizationOptions>(options =>
         {

src/modules/Elsa.Workflows.Api/Security/PythonWorkflowDefinitionAuthorizationService.cs

@@ -1,78 +0,0 @@
-using System.Security.Claims;
-using Elsa.Expressions.Contracts;
-using Elsa.Extensions;
-using Elsa.Workflows.Activities;
-using Elsa.Workflows.Management.Models;
-
-namespace Elsa.Workflows.Api.Security;
-
-internal class PythonWorkflowDefinitionAuthorizationService(
-    IActivityVisitor activityVisitor,
-    IExpressionDescriptorRegistry expressionDescriptorRegistry)
-{
-    public const string HostDisabledMessage = "Python.NET workflow expression execution is disabled by the host. Set PythonOptions.AllowHostCodeExecution to true only for trusted workflow authors; Python.NET is not a sandbox.";
-    private const string PythonExpressionType = "Python";
-    // Keep in sync with ActivityTypeNameHelper.GenerateTypeName<Elsa.Expressions.Python.Activities.RunPython>().
-    private const string RunPythonActivityType = "Elsa.RunPython";
-    private const string PermissionsClaimType = "permissions";
-
-    public async Task<PythonWorkflowDefinitionAuthorizationResult> AuthorizeAsync(WorkflowDefinitionModel model, ClaimsPrincipal user, CancellationToken cancellationToken = default)
-    {
-        if (model.Root == null || !await UsesPythonAsync(model.Root, cancellationToken))
-            return PythonWorkflowDefinitionAuthorizationResult.Allowed;
-
-        return AuthorizePythonUsage(user);
-    }
-
-    public async Task<PythonWorkflowDefinitionAuthorizationResult> AuthorizeAsync(Workflow workflow, ClaimsPrincipal user, CancellationToken cancellationToken = default)
-    {
-        if (!await UsesPythonAsync(workflow, cancellationToken))
-            return PythonWorkflowDefinitionAuthorizationResult.Allowed;
-
-        return AuthorizePythonUsage(user);
-    }
-
-    private PythonWorkflowDefinitionAuthorizationResult AuthorizePythonUsage(ClaimsPrincipal user)
-    {
-        // PythonOptions lives in the optional Python module. Workflows.Api observes the descriptor state projected by that module's provider.
-        if (expressionDescriptorRegistry.Find(PythonExpressionType)?.IsBrowsable != true)
-            return PythonWorkflowDefinitionAuthorizationResult.HostDisabled;
-
-        return HasPermission(user, PermissionNames.ExecutePythonExpressions)
-            ? PythonWorkflowDefinitionAuthorizationResult.Allowed
-            : PythonWorkflowDefinitionAuthorizationResult.MissingPermission;
-    }
-
-    private async Task<bool> UsesPythonAsync(IActivity root, CancellationToken cancellationToken)
-    {
-        var graph = await activityVisitor.VisitAsync(root, cancellationToken);
-        var nodes = new[] { graph }.Concat(graph.Descendants());
-
-        return nodes.Any(x => IsRunPythonActivity(x.Activity) || HasPythonExpression(x.Activity));
-    }
-
-    private static bool IsRunPythonActivity(IActivity activity)
-    {
-        return string.Equals(activity.Type, RunPythonActivityType, StringComparison.Ordinal);
-    }
-
-    private static bool HasPythonExpression(IActivity activity)
-    {
-        return activity.GetInputs().Any(x => string.Equals(x.Expression?.Type, PythonExpressionType, StringComparison.Ordinal));
-    }
-
-    private static bool HasPermission(ClaimsPrincipal user, string permission)
-    {
-        return user.Claims.Any(x =>
-            x.Type == PermissionsClaimType &&
-            (string.Equals(x.Value, PermissionNames.All, StringComparison.Ordinal) ||
-             string.Equals(x.Value, permission, StringComparison.Ordinal)));
-    }
-}
-
-internal enum PythonWorkflowDefinitionAuthorizationResult
-{
-    Allowed,
-    HostDisabled,
-    MissingPermission
-}

src/modules/Elsa.Workflows.Api/Security/WorkflowDefinitionScriptAuthorizationFailure.cs

@@ -1,21 +1,21 @@
 namespace Elsa.Workflows.Api.Security;
 
-internal static class PythonWorkflowDefinitionAuthorizationFailure
+internal static class WorkflowDefinitionScriptAuthorizationFailure
 {
     public static async Task SendAsync(
-        PythonWorkflowDefinitionAuthorizationResult result,
+        WorkflowDefinitionScriptAuthorizationResult result,
         Func<CancellationToken, Task> sendForbiddenAsync,
         Action<string> addError,
         Func<int, CancellationToken, Task> sendErrorsAsync,
         CancellationToken cancellationToken)
     {
-        if (result == PythonWorkflowDefinitionAuthorizationResult.MissingPermission)
+        if (result.FailureReason == WorkflowDefinitionScriptAuthorizationFailureReason.MissingPermission)
         {
             await sendForbiddenAsync(cancellationToken);
             return;
         }
 
-        addError(PythonWorkflowDefinitionAuthorizationService.HostDisabledMessage);
+        addError(result.Message ?? "Workflow script authorization failed.");
         await sendErrorsAsync(400, cancellationToken);
     }
 }

src/modules/Elsa.Workflows.Api/Security/WorkflowDefinitionScriptAuthorizationService.cs

@@ -0,0 +1,106 @@
+using System.Security.Claims;
+using Elsa.Expressions.Contracts;
+using Elsa.Extensions;
+using Elsa.Workflows.Activities;
+using Elsa.Workflows.Management.Models;
+
+namespace Elsa.Workflows.Api.Security;
+
+internal class WorkflowDefinitionScriptAuthorizationService(
+    IActivityVisitor activityVisitor,
+    IExpressionDescriptorRegistry expressionDescriptorRegistry)
+{
+    private static readonly ScriptPolicy[] ScriptPolicies =
+    [
+        new(
+            "CSharp",
+            WorkflowScriptActivityTypeNames.RunCSharp,
+            PermissionNames.ExecuteCSharpExpressions,
+            "C# workflow expression execution is disabled by the host. Set CSharpOptions.AllowHostCodeExecution to true only for trusted workflow authors; Roslyn scripting is not a sandbox."),
+        new(
+            "Python",
+            WorkflowScriptActivityTypeNames.RunPython,
+            PermissionNames.ExecutePythonExpressions,
+            "Python.NET workflow expression execution is disabled by the host. Set PythonOptions.AllowHostCodeExecution to true only for trusted workflow authors; Python.NET is not a sandbox.")
+    ];
+
+    public async Task<WorkflowDefinitionScriptAuthorizationResult> AuthorizeAsync(WorkflowDefinitionModel model, ClaimsPrincipal user, CancellationToken cancellationToken = default)
+    {
+        if (model.Root == null)
+            return WorkflowDefinitionScriptAuthorizationResult.Allowed();
+
+        return await AuthorizeAsync(model.Root, user, cancellationToken);
+    }
+
+    public async Task<WorkflowDefinitionScriptAuthorizationResult> AuthorizeAsync(IActivity root, ClaimsPrincipal user, CancellationToken cancellationToken = default)
+    {
+        var scriptUsages = await GetUsedScriptPoliciesAsync(root, cancellationToken);
+
+        var failure = scriptUsages
+            .Select(policy => AuthorizeScriptUsage(policy, user))
+            .FirstOrDefault(result => result is { Succeeded: false });
+
+        if (failure.FailureReason.HasValue)
+            return failure;
+
+        return WorkflowDefinitionScriptAuthorizationResult.Allowed();
+    }
+
+    public async Task<WorkflowDefinitionScriptAuthorizationResult> AuthorizeAsync(Workflow workflow, ClaimsPrincipal user, CancellationToken cancellationToken = default)
+    {
+        return await AuthorizeAsync((IActivity)workflow, user, cancellationToken);
+    }
+
+    private WorkflowDefinitionScriptAuthorizationResult AuthorizeScriptUsage(ScriptPolicy policy, ClaimsPrincipal user)
+    {
+        // Language-specific options live in optional modules. Workflows.Api observes the descriptor state projected by those module providers.
+        if (expressionDescriptorRegistry.Find(policy.ExpressionType)?.IsBrowsable != true)
+            return WorkflowDefinitionScriptAuthorizationResult.HostDisabled(policy.HostDisabledMessage);
+
+        return HasPermission(user, policy.Permission)
+            ? WorkflowDefinitionScriptAuthorizationResult.Allowed()
+            : WorkflowDefinitionScriptAuthorizationResult.MissingPermission();
+    }
+
+    private async Task<IEnumerable<ScriptPolicy>> GetUsedScriptPoliciesAsync(IActivity root, CancellationToken cancellationToken)
+    {
+        var graph = await activityVisitor.VisitAsync(root, cancellationToken);
+        var nodes = new[] { graph }.Concat(graph.Descendants()).ToList();
+        var policies = ScriptPolicies
+            .Where(policy => nodes.Any(x => IsRunActivity(x.Activity, policy) || HasExpression(x.Activity, policy)))
+            .ToList();
+
+        return policies;
+    }
+
+    private static bool IsRunActivity(IActivity activity, ScriptPolicy policy) =>
+        string.Equals(activity.Type, policy.RunActivityType, StringComparison.Ordinal);
+
+    private static bool HasExpression(IActivity activity, ScriptPolicy policy) =>
+        activity.GetInputs().Any(x => string.Equals(x.Expression?.Type, policy.ExpressionType, StringComparison.Ordinal));
+
+    private static bool HasPermission(ClaimsPrincipal user, string permission)
+    {
+        return user.Claims.Any(x =>
+            x.Type == PermissionNames.ClaimType &&
+            (string.Equals(x.Value, PermissionNames.All, StringComparison.Ordinal) ||
+             string.Equals(x.Value, permission, StringComparison.Ordinal)));
+    }
+
+    private sealed record ScriptPolicy(string ExpressionType, string RunActivityType, string Permission, string HostDisabledMessage);
+}
+
+internal readonly record struct WorkflowDefinitionScriptAuthorizationResult(bool Succeeded, WorkflowDefinitionScriptAuthorizationFailureReason? FailureReason, string? Message)
+{
+    public static WorkflowDefinitionScriptAuthorizationResult Allowed() => new(true, null, null);
+
+    public static WorkflowDefinitionScriptAuthorizationResult HostDisabled(string message) => new(false, WorkflowDefinitionScriptAuthorizationFailureReason.HostDisabled, message);
+
+    public static WorkflowDefinitionScriptAuthorizationResult MissingPermission() => new(false, WorkflowDefinitionScriptAuthorizationFailureReason.MissingPermission, null);
+}
+
+internal enum WorkflowDefinitionScriptAuthorizationFailureReason
+{
+    HostDisabled,
+    MissingPermission
+}

src/modules/Elsa.Workflows.Api/ShellFeatures/WorkflowsApiFeature.cs

@@ -37,7 +37,7 @@ public class WorkflowsApiFeature : IFastEndpointsShellFeature
     {
         services.AddSerializationOptionsConfigurator<SerializationConfigurator>();
         services.AddScoped<IWorkflowDefinitionLinker, StaticWorkflowDefinitionLinker>();
-        services.AddScoped<PythonWorkflowDefinitionAuthorizationService>();
+        services.AddScoped<WorkflowDefinitionScriptAuthorizationService>();
         services.AddScoped<IAuthorizationHandler, NotReadOnlyRequirementHandler>();
         services.Configure<AuthorizationOptions>(options =>
         {

src/modules/Elsa.Workflows.Core/Constants/WorkflowScriptActivityTypeNames.cs

@@ -0,0 +1,32 @@
+namespace Elsa.Workflows;
+
+/// <summary>
+/// Activity type names for built-in workflow script activities.
+/// </summary>
+public static class WorkflowScriptActivityTypeNames
+{
+    /// <summary>
+    /// The Elsa activity namespace for built-in script activities.
+    /// </summary>
+    public const string Namespace = "Elsa";
+
+    /// <summary>
+    /// The unqualified C# script activity type.
+    /// </summary>
+    public const string RunCSharpType = "RunCSharp";
+
+    /// <summary>
+    /// The fully qualified C# script activity type name.
+    /// </summary>
+    public const string RunCSharp = $"{Namespace}.{RunCSharpType}";
+
+    /// <summary>
+    /// The unqualified Python script activity type.
+    /// </summary>
+    public const string RunPythonType = "RunPython";
+
+    /// <summary>
+    /// The fully qualified Python script activity type name.
+    /// </summary>
+    public const string RunPython = $"{Namespace}.{RunPythonType}";
+}

test/integration/Elsa.Workflows.IntegrationTests/Security/PythonWorkflowDefinitionAuthorizationServiceTests.cs

@@ -1,110 +0,0 @@
-using System.Security.Claims;
-using Elsa.Expressions.Contracts;
-using Elsa.Expressions.Models;
-using Elsa.Workflows.Activities;
-using Elsa.Workflows.Api.Security;
-using Elsa.Workflows.Management.Models;
-using Elsa.Workflows.Management.Services;
-using Elsa.Workflows.Models;
-using Elsa.Workflows.PortResolvers;
-using Microsoft.Extensions.DependencyInjection;
-using NSubstitute;
-
-namespace Elsa.Workflows.IntegrationTests.Security;
-
-public class PythonWorkflowDefinitionAuthorizationServiceTests
-{
-    private static readonly ClaimsPrincipal UserWithPythonPermission = CreateUser(PermissionNames.ExecutePythonExpressions);
-    private static readonly ClaimsPrincipal UserWithoutPythonPermission = CreateUser("write:workflow-definitions");
-
-    [Fact]
-    public async Task AuthorizeAsync_BlocksPythonExpression_WhenHostHasNotOptedIn()
-    {
-        var service = CreateService(hostAllowsPython: false);
-        var model = CreateModelWithPythonExpression();
-
-        var result = await service.AuthorizeAsync(model, UserWithPythonPermission);
-
-        Assert.Equal(PythonWorkflowDefinitionAuthorizationResult.HostDisabled, result);
-    }
-
-    [Fact]
-    public async Task AuthorizeAsync_BlocksPythonExpression_WhenUserLacksPermission()
-    {
-        var service = CreateService(hostAllowsPython: true);
-        var model = CreateModelWithPythonExpression();
-
-        var result = await service.AuthorizeAsync(model, UserWithoutPythonPermission);
-
-        Assert.Equal(PythonWorkflowDefinitionAuthorizationResult.MissingPermission, result);
-    }
-
-    [Fact]
-    public async Task AuthorizeAsync_AllowsPythonExpression_WhenHostAndUserAllowIt()
-    {
-        var service = CreateService(hostAllowsPython: true);
-        var model = CreateModelWithPythonExpression();
-
-        var result = await service.AuthorizeAsync(model, UserWithPythonPermission);
-
-        Assert.Equal(PythonWorkflowDefinitionAuthorizationResult.Allowed, result);
-    }
-
-    [Fact]
-    public async Task AuthorizeAsync_TreatsRunPythonActivityAsPythonUsage()
-    {
-        var service = CreateService(hostAllowsPython: true);
-        var model = new WorkflowDefinitionModel
-        {
-            Root = new WriteLine("hello")
-            {
-                Type = "Elsa.RunPython"
-            }
-        };
-
-        var result = await service.AuthorizeAsync(model, UserWithoutPythonPermission);
-
-        Assert.Equal(PythonWorkflowDefinitionAuthorizationResult.MissingPermission, result);
-    }
-
-    private static WorkflowDefinitionModel CreateModelWithPythonExpression()
-    {
-        return new()
-        {
-            Root = new WriteLine("placeholder")
-            {
-                Text = new Input<string>(new Expression("Python", "'hello'"))
-            }
-        };
-    }
-
-    private static PythonWorkflowDefinitionAuthorizationService CreateService(bool hostAllowsPython)
-    {
-        var expressionDescriptor = new ExpressionDescriptor
-        {
-            Type = "Python",
-            DisplayName = "Python",
-            IsBrowsable = hostAllowsPython,
-            HandlerFactory = _ => Substitute.For<IExpressionHandler>()
-        };
-
-        var provider = Substitute.For<IExpressionDescriptorProvider>();
-        provider.GetDescriptors().Returns([expressionDescriptor]);
-
-        var registry = new ExpressionDescriptorRegistry([provider]);
-        var visitor = new ActivityVisitor(
-            [
-                new SwitchActivityResolver(),
-                new PropertyBasedActivityResolver()
-            ],
-            new ServiceCollection().BuildServiceProvider());
-
-        return new(visitor, registry);
-    }
-
-    private static ClaimsPrincipal CreateUser(params string[] permissions)
-    {
-        var identity = new ClaimsIdentity(permissions.Select(x => new Claim("permissions", x)), "Test");
-        return new(identity);
-    }
-}

test/integration/Elsa.Workflows.IntegrationTests/Security/WorkflowDefinitionScriptAuthorizationServiceTests.cs

@@ -0,0 +1,198 @@
+using System.Security.Claims;
+using Elsa.Expressions.Contracts;
+using Elsa.Expressions.Models;
+using Elsa.Workflows.Activities;
+using Elsa.Workflows.Api.Security;
+using Elsa.Workflows.Management.Models;
+using Elsa.Workflows.Management.Services;
+using Elsa.Workflows.Models;
+using Elsa.Workflows.PortResolvers;
+using Microsoft.Extensions.DependencyInjection;
+using NSubstitute;
+
+namespace Elsa.Workflows.IntegrationTests.Security;
+
+public class WorkflowDefinitionScriptAuthorizationServiceTests
+{
+    private static readonly ClaimsPrincipal UserWithCSharpPermission = CreateUser(PermissionNames.ExecuteCSharpExpressions);
+    private static readonly ClaimsPrincipal UserWithPythonPermission = CreateUser(PermissionNames.ExecutePythonExpressions);
+    private static readonly ClaimsPrincipal UserWithoutScriptPermission = CreateUser("write:workflow-definitions");
+
+    [Fact]
+    public async Task AuthorizeAsync_BlocksCSharpExpression_WhenHostHasNotOptedIn()
+    {
+        var service = CreateService(hostAllowsCSharp: false, hostAllowsPython: true);
+        var model = CreateModelWithCSharpExpression();
+
+        var result = await service.AuthorizeAsync(model, UserWithCSharpPermission);
+
+        Assert.Equal(WorkflowDefinitionScriptAuthorizationFailureReason.HostDisabled, result.FailureReason);
+        Assert.Contains("CSharpOptions.AllowHostCodeExecution", result.Message);
+    }
+
+    [Fact]
+    public async Task AuthorizeAsync_BlocksCSharpExpression_WhenUserLacksPermission()
+    {
+        var service = CreateService(hostAllowsCSharp: true, hostAllowsPython: true);
+        var model = CreateModelWithCSharpExpression();
+
+        var result = await service.AuthorizeAsync(model, UserWithoutScriptPermission);
+
+        Assert.Equal(WorkflowDefinitionScriptAuthorizationFailureReason.MissingPermission, result.FailureReason);
+    }
+
+    [Fact]
+    public async Task AuthorizeAsync_AllowsCSharpExpression_WhenHostAndUserAllowIt()
+    {
+        var service = CreateService(hostAllowsCSharp: true, hostAllowsPython: true);
+        var model = CreateModelWithCSharpExpression();
+
+        var result = await service.AuthorizeAsync(model, UserWithCSharpPermission);
+
+        Assert.True(result.Succeeded);
+    }
+
+    [Fact]
+    public async Task AuthorizeAsync_AllowsWorkflowWithoutScriptUsage()
+    {
+        var service = CreateService(hostAllowsCSharp: true, hostAllowsPython: true);
+        var model = new WorkflowDefinitionModel
+        {
+            Root = new WriteLine("hello")
+        };
+
+        var result = await service.AuthorizeAsync(model, UserWithoutScriptPermission);
+
+        Assert.True(result.Succeeded);
+    }
+
+    [Fact]
+    public async Task AuthorizeAsync_TreatsRunCSharpActivityAsCSharpUsage()
+    {
+        var service = CreateService(hostAllowsCSharp: true, hostAllowsPython: true);
+        var model = new WorkflowDefinitionModel
+        {
+            Root = new WriteLine("hello")
+            {
+                Type = WorkflowScriptActivityTypeNames.RunCSharp
+            }
+        };
+
+        var result = await service.AuthorizeAsync(model, UserWithoutScriptPermission);
+
+        Assert.Equal(WorkflowDefinitionScriptAuthorizationFailureReason.MissingPermission, result.FailureReason);
+    }
+
+    [Fact]
+    public async Task AuthorizeAsync_BlocksPythonExpression_WhenHostHasNotOptedIn()
+    {
+        var service = CreateService(hostAllowsCSharp: true, hostAllowsPython: false);
+        var model = CreateModelWithPythonExpression();
+
+        var result = await service.AuthorizeAsync(model, UserWithPythonPermission);
+
+        Assert.Equal(WorkflowDefinitionScriptAuthorizationFailureReason.HostDisabled, result.FailureReason);
+        Assert.Contains("PythonOptions.AllowHostCodeExecution", result.Message);
+    }
+
+    [Fact]
+    public async Task AuthorizeAsync_BlocksPythonExpression_WhenUserLacksPermission()
+    {
+        var service = CreateService(hostAllowsCSharp: true, hostAllowsPython: true);
+        var model = CreateModelWithPythonExpression();
+
+        var result = await service.AuthorizeAsync(model, UserWithoutScriptPermission);
+
+        Assert.Equal(WorkflowDefinitionScriptAuthorizationFailureReason.MissingPermission, result.FailureReason);
+    }
+
+    [Fact]
+    public async Task AuthorizeAsync_AllowsPythonExpression_WhenHostAndUserAllowIt()
+    {
+        var service = CreateService(hostAllowsCSharp: true, hostAllowsPython: true);
+        var model = CreateModelWithPythonExpression();
+
+        var result = await service.AuthorizeAsync(model, UserWithPythonPermission);
+
+        Assert.True(result.Succeeded);
+    }
+
+    [Fact]
+    public async Task AuthorizeAsync_TreatsRunPythonActivityAsPythonUsage()
+    {
+        var service = CreateService(hostAllowsCSharp: true, hostAllowsPython: true);
+        var model = new WorkflowDefinitionModel
+        {
+            Root = new WriteLine("hello")
+            {
+                Type = WorkflowScriptActivityTypeNames.RunPython
+            }
+        };
+
+        var result = await service.AuthorizeAsync(model, UserWithoutScriptPermission);
+
+        Assert.Equal(WorkflowDefinitionScriptAuthorizationFailureReason.MissingPermission, result.FailureReason);
+    }
+
+    private static WorkflowDefinitionModel CreateModelWithCSharpExpression()
+    {
+        return new()
+        {
+            Root = new WriteLine("placeholder")
+            {
+                Text = new Input<string>(new Expression("CSharp", "\"hello\""))
+            }
+        };
+    }
+
+    private static WorkflowDefinitionModel CreateModelWithPythonExpression()
+    {
+        return new()
+        {
+            Root = new WriteLine("placeholder")
+            {
+                Text = new Input<string>(new Expression("Python", "'hello'"))
+            }
+        };
+    }
+
+    private static WorkflowDefinitionScriptAuthorizationService CreateService(bool hostAllowsCSharp, bool hostAllowsPython)
+    {
+        var expressionDescriptors = new[]
+        {
+            new ExpressionDescriptor
+            {
+                Type = "CSharp",
+                DisplayName = "C#",
+                IsBrowsable = hostAllowsCSharp,
+                HandlerFactory = _ => Substitute.For<IExpressionHandler>()
+            },
+            new ExpressionDescriptor
+            {
+                Type = "Python",
+                DisplayName = "Python",
+                IsBrowsable = hostAllowsPython,
+                HandlerFactory = _ => Substitute.For<IExpressionHandler>()
+            }
+        };
+
+        var provider = Substitute.For<IExpressionDescriptorProvider>();
+        provider.GetDescriptors().Returns(expressionDescriptors);
+
+        var registry = new ExpressionDescriptorRegistry([provider]);
+        var visitor = new ActivityVisitor(
+            [
+                new SwitchActivityResolver(),
+                new PropertyBasedActivityResolver()
+            ],
+            new ServiceCollection().BuildServiceProvider());
+
+        return new(visitor, registry);
+    }
+
+    private static ClaimsPrincipal CreateUser(params string[] permissions)
+    {
+        var identity = new ClaimsIdentity(permissions.Select(x => new Claim("permissions", x)), "Test");
+        return new(identity);
+    }
+}

test/unit/Elsa.Expressions.UnitTests/CSharp/CSharpHostCodeExecutionTests.cs

@@ -0,0 +1,54 @@
+using Elsa.Expressions.Contracts;
+using Elsa.Expressions.CSharp.Contracts;
+using Elsa.Expressions.CSharp.Options;
+using Elsa.Expressions.CSharp.Services;
+using Elsa.Expressions.Models;
+using Elsa.Testing.Shared;
+using Elsa.Workflows.Activities;
+using Microsoft.Extensions.Caching.Memory;
+using Microsoft.Extensions.DependencyInjection;
+using NSubstitute;
+
+namespace Elsa.Expressions.UnitTests.CSharp;
+
+public class CSharpHostCodeExecutionTests
+{
+    [Fact]
+    public async Task Evaluator_BlocksExecution_WhenHostHasNotOptedIn()
+    {
+        using var memoryCache = new MemoryCache(new MemoryCacheOptions());
+        var evaluator = new CSharpEvaluator(
+            Substitute.For<Elsa.Mediator.Contracts.INotificationSender>(),
+            Microsoft.Extensions.Options.Options.Create(new CSharpOptions()),
+            memoryCache);
+        var context = await new ActivityTestFixture(new WriteLine("test")).BuildAsync();
+
+        var exception = await Assert.ThrowsAsync<InvalidOperationException>(() =>
+            evaluator.EvaluateAsync("\"hello\"", typeof(string), context.ExpressionExecutionContext, new ExpressionEvaluatorOptions()));
+
+        Assert.Contains(nameof(CSharpOptions.AllowHostCodeExecution), exception.Message);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void Descriptor_Browsability_FollowsHostOptIn(bool allowHostCodeExecution)
+    {
+        var services = new ServiceCollection();
+        services.AddOptions();
+        services.AddMemoryCache();
+        new Elsa.Expressions.CSharp.ShellFeatures.CSharpFeature
+        {
+            CSharpOptions = options => options.AllowHostCodeExecution = allowHostCodeExecution
+        }.ConfigureServices(services);
+        services.AddSingleton<IExpressionDescriptorRegistry, Elsa.Workflows.Management.Services.ExpressionDescriptorRegistry>();
+
+        var serviceProvider = services.BuildServiceProvider();
+        var registry = serviceProvider.GetRequiredService<IExpressionDescriptorRegistry>();
+
+        var descriptor = registry.Find("CSharp");
+
+        Assert.NotNull(descriptor);
+        Assert.Equal(allowHostCodeExecution, descriptor.IsBrowsable);
+    }
+}