Add CORS to OTLP HTTP endpoint

src/Aspire.Dashboard/Configuration/DashboardOptions.cs

@@ -94,6 +94,8 @@ public byte[] GetPrimaryApiKeyBytes()
 
     public byte[]? GetSecondaryApiKeyBytes() => _secondaryApiKeyBytes;
 
+    public OtlpCors Cors { get; set; } = new();
+
     internal bool TryParseOptions([NotNullWhen(false)] out string? errorMessage)
     {
         if (string.IsNullOrEmpty(GrpcEndpointUrl) && string.IsNullOrEmpty(HttpEndpointUrl))
@@ -114,6 +116,12 @@ internal bool TryParseOptions([NotNullWhen(false)] out string? errorMessage)
             return false;
         }
 
+        if (string.IsNullOrEmpty(HttpEndpointUrl) && !string.IsNullOrEmpty(Cors.AllowedOrigins))
+        {
+            errorMessage = $"CORS configured without an OTLP HTTP endpoint. Either remove CORS configuration or specify a {DashboardConfigNames.DashboardOtlpHttpUrlName.EnvVarName} value.";
+            return false;
+        }
+
         _primaryApiKeyBytes = PrimaryApiKey != null ? Encoding.UTF8.GetBytes(PrimaryApiKey) : null;
         _secondaryApiKeyBytes = SecondaryApiKey != null ? Encoding.UTF8.GetBytes(SecondaryApiKey) : null;
 
@@ -122,6 +130,12 @@ internal bool TryParseOptions([NotNullWhen(false)] out string? errorMessage)
     }
 }
 
+public sealed class OtlpCors
+{
+    public string? AllowedOrigins { get; set; }
+    public string? AllowedHeaders { get; set; }
+}
+
 // Don't set values after validating/parsing options.
 public sealed class FrontendOptions
 {

src/Aspire.Dashboard/DashboardWebApplication.cs

@@ -148,6 +148,7 @@ public DashboardWebApplication(
             // See https://learn.microsoft.com/aspnet/core/performance/response-compression#compression-with-https for more information
             options.MimeTypes = ["text/javascript", "application/javascript", "text/css", "image/svg+xml"];
         });
+        builder.Services.AddCors();
 
         // Data from the server.
         builder.Services.AddScoped<IDashboardClient, DashboardClient>();
@@ -257,6 +258,13 @@ public DashboardWebApplication(
             await next(context).ConfigureAwait(false);
         });
 
+        if (!string.IsNullOrEmpty(dashboardOptions.Otlp.Cors.AllowedOrigins))
+        {
+            // Only add CORS middleware when there is CORS configuration.
+            // Because there isn't a default policy, CORS isn't enabled expect on certain endpoints, e.g. OTLP HTTP endpoints.
+            _app.UseCors();
+        }
+
         _app.UseMiddleware<ValidateTokenMiddleware>();
 
         // Configure the HTTP request pipeline.
@@ -301,11 +309,7 @@ public DashboardWebApplication(
         _app.MapRazorComponents<App>().AddInteractiveServerRenderMode();
 
         // OTLP HTTP services.
-        var httpEndpoint = dashboardOptions.Otlp.GetHttpEndpointUri();
-        if (httpEndpoint != null)
-        {
-            _app.MapHttpOtlpApi();
-        }
+        _app.MapHttpOtlpApi(dashboardOptions.Otlp);
 
         // OTLP gRPC services.
         _app.MapGrpcService<OtlpGrpcMetricsService>();
@@ -697,13 +701,13 @@ public int Run()
 
     public Task StartAsync(CancellationToken cancellationToken = default)
     {
-        Debug.Assert(_validationFailures.Count == 0);
+        Debug.Assert(_validationFailures.Count == 0, "Validation failures: " + Environment.NewLine + string.Join(Environment.NewLine, _validationFailures));
         return _app.StartAsync(cancellationToken);
     }
 
     public Task StopAsync(CancellationToken cancellationToken = default)
     {
-        Debug.Assert(_validationFailures.Count == 0);
+        Debug.Assert(_validationFailures.Count == 0, "Validation failures: " + Environment.NewLine + string.Join(Environment.NewLine, _validationFailures));
         return _app.StopAsync(cancellationToken);
     }
 

src/Aspire.Dashboard/Otlp/Http/OtlpHttpEndpointsBuilder.cs

@@ -6,6 +6,7 @@
 using System.Net.Http.Headers;
 using System.Reflection;
 using Aspire.Dashboard.Authentication;
+using Aspire.Dashboard.Configuration;
 using Google.Protobuf;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.Extensions.Primitives;
@@ -19,13 +20,41 @@ public static class OtlpHttpEndpointsBuilder
 {
     public const string ProtobufContentType = "application/x-protobuf";
     public const string JsonContentType = "application/json";
+    // By default, allow headers in the implicit safelist and X-Requested-With. This matches OTLP collector CORS behavior.
+    // Implicit safelist: https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_request_header
+    // OTLP collector: https://github.com/open-telemetry/opentelemetry-collector/blob/685625abb4703cb2e45a397f008127bbe2ba4c0e/config/confighttp/README.md#server-configuration
+    public static readonly string[] DefaultAllowedHeaders = ["X-Requested-With"];
 
-    public static void MapHttpOtlpApi(this IEndpointRouteBuilder endpoints)
+    public static void MapHttpOtlpApi(this IEndpointRouteBuilder endpoints, OtlpOptions options)
     {
+        var httpEndpoint = options.GetHttpEndpointUri();
+        if (httpEndpoint == null)
+        {
+            // Don't map OTLP HTTP route endpoints if there isn't a Kestrel endpoint to access them with.
+            return;
+        }
+
         var group = endpoints
             .MapGroup("/v1")
             .AddOtlpHttpMetadata();
 
+        if (!string.IsNullOrEmpty(options.Cors.AllowedOrigins))
+        {
+            group = group.RequireCors(builder =>
+            {
+                builder.WithOrigins(options.Cors.AllowedOrigins.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries));
+                builder.SetIsOriginAllowedToAllowWildcardSubdomains();
+
+                var allowedHeaders = !string.IsNullOrEmpty(options.Cors.AllowedHeaders)
+                    ? options.Cors.AllowedHeaders.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+                    : DefaultAllowedHeaders;
+                builder.WithHeaders(allowedHeaders);
+
+                // Hardcode to allow only POST methods. OTLP is always sent in POST request bodies.
+                builder.WithMethods(HttpMethods.Post);
+            });
+        }
+
         group.MapPost("logs", static (MessageBindable<ExportLogsServiceRequest> request, OtlpLogsService service) =>
         {
             if (request.Message == null)

src/Shared/DashboardConfigNames.cs

@@ -15,6 +15,8 @@ internal static class DashboardConfigNames
     public static readonly ConfigName DashboardOtlpAuthModeName = new("Dashboard:Otlp:AuthMode", "DASHBOARD__OTLP__AUTHMODE");
     public static readonly ConfigName DashboardOtlpPrimaryApiKeyName = new("Dashboard:Otlp:PrimaryApiKey", "DASHBOARD__OTLP__PRIMARYAPIKEY");
     public static readonly ConfigName DashboardOtlpSecondaryApiKeyName = new("Dashboard:Otlp:SecondaryApiKey", "DASHBOARD__OTLP__SECONDARYAPIKEY");
+    public static readonly ConfigName DashboardOtlpCorsAllowedOriginsKeyName = new("Dashboard:Otlp:Cors:AllowedOrigins", "DASHBOARD__OTLP__CORS__ALLOWEDORIGINS");
+    public static readonly ConfigName DashboardOtlpCorsAllowedHeadersKeyName = new("Dashboard:Otlp:Cors:AllowedHeaders", "DASHBOARD__OTLP__CORS__ALLOWEDHEADERS");
     public static readonly ConfigName DashboardFrontendAuthModeName = new("Dashboard:Frontend:AuthMode", "DASHBOARD__FRONTEND__AUTHMODE");
     public static readonly ConfigName DashboardFrontendBrowserTokenName = new("Dashboard:Frontend:BrowserToken", "DASHBOARD__FRONTEND__BROWSERTOKEN");
     public static readonly ConfigName DashboardFrontendMaxConsoleLogCountName = new("Dashboard:Frontend:MaxConsoleLogCount", "DASHBOARD__FRONTEND__MAXCONSOLELOGCOUNT");

tests/Aspire.Dashboard.Tests/Integration/OtlpCorsHttpServiceTests.cs

@@ -0,0 +1,136 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Net;
+using Aspire.Hosting;
+using Xunit;
+using Xunit.Abstractions;
+
+namespace Aspire.Dashboard.Tests.Integration;
+
+public class OtlpCorsHttpServiceTests
+{
+    private readonly ITestOutputHelper _testOutputHelper;
+
+    public OtlpCorsHttpServiceTests(ITestOutputHelper testOutputHelper)
+    {
+        _testOutputHelper = testOutputHelper;
+    }
+
+    [Fact]
+    public async Task ReceivePreflight_OtlpHttpEndPoint_NoCorsConfiguration_NotFound()
+    {
+        // Arrange
+        await using var app = IntegrationTestHelpers.CreateDashboardWebApplication(_testOutputHelper);
+        await app.StartAsync();
+
+        using var httpClient = IntegrationTestHelpers.CreateHttpClient($"http://{app.OtlpServiceHttpEndPointAccessor().EndPoint}");
+
+        var preflightRequest = new HttpRequestMessage(HttpMethod.Options, "/v1/logs");
+        preflightRequest.Headers.TryAddWithoutValidation("Access-Control-Request-Method", "POST");
+        preflightRequest.Headers.TryAddWithoutValidation("Access-Control-Request-Headers", "x-requested-with,x-custom,Content-Type");
+        preflightRequest.Headers.TryAddWithoutValidation("Origin", "http://localhost:8000");
+
+        // Act
+        var responseMessage = await httpClient.SendAsync(preflightRequest);
+
+        // Assert
+        Assert.Equal(HttpStatusCode.NotFound, responseMessage.StatusCode);
+    }
+
+    [Fact]
+    public async Task ReceivePreflight_OtlpHttpEndPoint_ValidCorsOrigin_Success()
+    {
+        // Arrange
+        await using var app = IntegrationTestHelpers.CreateDashboardWebApplication(_testOutputHelper, config =>
+        {
+            config[DashboardConfigNames.DashboardOtlpCorsAllowedOriginsKeyName.ConfigKey] = "http://localhost:8000, http://localhost:8001";
+        });
+        await app.StartAsync();
+
+        using var httpClient = IntegrationTestHelpers.CreateHttpClient($"http://{app.OtlpServiceHttpEndPointAccessor().EndPoint}");
+
+        // Act 1
+        var preflightRequest1 = new HttpRequestMessage(HttpMethod.Options, "/v1/logs");
+        preflightRequest1.Headers.TryAddWithoutValidation("Access-Control-Request-Method", "POST");
+        preflightRequest1.Headers.TryAddWithoutValidation("Access-Control-Request-Headers", "x-requested-with,x-custom,Content-Type");
+        preflightRequest1.Headers.TryAddWithoutValidation("Origin", "http://localhost:8000");
+
+        var responseMessage1 = await httpClient.SendAsync(preflightRequest1);
+
+        // Assert 1
+        Assert.Equal(HttpStatusCode.NoContent, responseMessage1.StatusCode);
+        Assert.Equal("http://localhost:8000", responseMessage1.Headers.GetValues("Access-Control-Allow-Origin").Single());
+        Assert.Equal("POST", responseMessage1.Headers.GetValues("Access-Control-Allow-Methods").Single());
+        Assert.Equal("X-Requested-With", responseMessage1.Headers.GetValues("Access-Control-Allow-Headers").Single());
+
+        // Act 2
+        var preflightRequest2 = new HttpRequestMessage(HttpMethod.Options, "/v1/logs");
+        preflightRequest2.Headers.TryAddWithoutValidation("Access-Control-Request-Method", "POST");
+        preflightRequest2.Headers.TryAddWithoutValidation("Access-Control-Request-Headers", "x-requested-with,x-custom,Content-Type");
+        preflightRequest2.Headers.TryAddWithoutValidation("Origin", "http://localhost:8001");
+
+        var responseMessage2 = await httpClient.SendAsync(preflightRequest2);
+
+        // Assert 2
+        Assert.Equal(HttpStatusCode.NoContent, responseMessage2.StatusCode);
+        Assert.Equal("http://localhost:8001", responseMessage2.Headers.GetValues("Access-Control-Allow-Origin").Single());
+        Assert.Equal("POST", responseMessage2.Headers.GetValues("Access-Control-Allow-Methods").Single());
+        Assert.Equal("X-Requested-With", responseMessage2.Headers.GetValues("Access-Control-Allow-Headers").Single());
+    }
+
+    [Fact]
+    public async Task ReceivePreflight_OtlpHttpEndPoint_InvalidCorsOrigin_NoCorsHeadersReturned()
+    {
+        // Arrange
+        await using var app = IntegrationTestHelpers.CreateDashboardWebApplication(_testOutputHelper, config =>
+        {
+            config[DashboardConfigNames.DashboardOtlpCorsAllowedOriginsKeyName.ConfigKey] = "http://localhost:8000";
+        });
+        await app.StartAsync();
+
+        using var httpClient = IntegrationTestHelpers.CreateHttpClient($"http://{app.OtlpServiceHttpEndPointAccessor().EndPoint}");
+
+        var preflightRequest = new HttpRequestMessage(HttpMethod.Options, "/v1/logs");
+        preflightRequest.Headers.TryAddWithoutValidation("Access-Control-Request-Method", "POST");
+        preflightRequest.Headers.TryAddWithoutValidation("Access-Control-Request-Headers", "x-requested-with,x-custom,Content-Type");
+        preflightRequest.Headers.TryAddWithoutValidation("Origin", "http://localhost:8001");
+
+        // Act
+        var responseMessage = await httpClient.SendAsync(preflightRequest);
+
+        // Assert
+        Assert.Equal(HttpStatusCode.NoContent, responseMessage.StatusCode);
+        Assert.False(responseMessage.Headers.Contains("Access-Control-Allow-Origin"));
+        Assert.False(responseMessage.Headers.Contains("Access-Control-Allow-Methods"));
+        Assert.False(responseMessage.Headers.Contains("Access-Control-Allow-Headers"));
+    }
+
+    [Fact]
+    public async Task ReceivePreflight_OtlpHttpEndPoint_AnyOrigin_Success()
+    {
+        // Arrange
+        await using var app = IntegrationTestHelpers.CreateDashboardWebApplication(_testOutputHelper, config =>
+        {
+            config[DashboardConfigNames.DashboardOtlpCorsAllowedOriginsKeyName.ConfigKey] = "*";
+            config[DashboardConfigNames.DashboardOtlpCorsAllowedHeadersKeyName.ConfigKey] = "*";
+        });
+        await app.StartAsync();
+
+        using var httpClient = IntegrationTestHelpers.CreateHttpClient($"http://{app.OtlpServiceHttpEndPointAccessor().EndPoint}");
+
+        var preflightRequest = new HttpRequestMessage(HttpMethod.Options, "/v1/logs");
+        preflightRequest.Headers.TryAddWithoutValidation("Access-Control-Request-Method", "POST");
+        preflightRequest.Headers.TryAddWithoutValidation("Access-Control-Request-Headers", "x-requested-with,x-custom,Content-Type");
+        preflightRequest.Headers.TryAddWithoutValidation("Origin", "http://localhost:8000");
+
+        // Act
+        var responseMessage = await httpClient.SendAsync(preflightRequest);
+
+        // Assert
+        Assert.Equal(HttpStatusCode.NoContent, responseMessage.StatusCode);
+        Assert.Equal("*", responseMessage.Headers.GetValues("Access-Control-Allow-Origin").Single());
+        Assert.Equal("POST", responseMessage.Headers.GetValues("Access-Control-Allow-Methods").Single());
+        Assert.Equal("x-requested-with,x-custom,Content-Type", responseMessage.Headers.GetValues("Access-Control-Allow-Headers").Single());
+    }
+}

tests/Aspire.Dashboard.Tests/Integration/StartupTests.cs

@@ -436,6 +436,22 @@ public async Task EndPointAccessors_AppStarted_BrowserGet_Success()
         Assert.NotEmpty(response.Headers.GetValues(HeaderNames.ContentSecurityPolicy).Single());
     }
 
+    [Fact]
+    public async Task Configuration_CorsNoOtlpHttpEndpoint_Error()
+    {
+        // Arrange & Act
+        await using var app = IntegrationTestHelpers.CreateDashboardWebApplication(testOutputHelper,
+            additionalConfiguration: data =>
+            {
+                data.Remove(DashboardConfigNames.DashboardOtlpHttpUrlName.ConfigKey);
+                data[DashboardConfigNames.DashboardOtlpCorsAllowedOriginsKeyName.ConfigKey] = "https://localhost:666";
+            });
+
+        // Assert
+        Assert.Collection(app.ValidationFailures,
+            s => Assert.Contains(DashboardConfigNames.DashboardOtlpHttpUrlName.ConfigKey, s));
+    }
+
     private static void AssertDynamicIPEndpoint(Func<EndpointInfo> endPointAccessor)
     {
         // Check that the specified dynamic port of 0 is overridden with the actual port number.