[backend] Reject requests if Host header doesn't match configured WebDomain or AccountDomain

Iceshrimp.Backend/Core/Configuration/Config.cs

@@ -31,6 +31,7 @@ public sealed class Config {
 		public string UserAgent => $"Iceshrimp.NET/{Version} (https://{WebDomain})";
 
 		public required int    ListenPort    { get; init; } = 3000;
+		public required string ListenHost    { get; init; } = "localhost";
 		public required string WebDomain     { get; init; }
 		public required string AccountDomain { get; init; }
 	}

Iceshrimp.Backend/Core/Extensions/ServiceExtensions.cs

@@ -47,6 +47,7 @@ public static class ServiceExtensions {
 		services.AddSingleton<RequestBufferingMiddleware>();
 		services.AddSingleton<AuthorizationMiddleware>();
 		services.AddSingleton<OauthAuthorizationMiddleware>();
+		services.AddSingleton<RequestVerificationMiddleware>();
 
 		// Hosted services = long running background tasks
 		// Note: These need to be added as a singleton as well to ensure data consistency

Iceshrimp.Backend/Core/Extensions/WebApplicationExtensions.cs

@@ -10,6 +10,7 @@ public static class WebApplicationExtensions {
 	public static IApplicationBuilder UseCustomMiddleware(this IApplicationBuilder app) {
 		// Caution: make sure these are in the correct order
 		return app.UseMiddleware<ErrorHandlerMiddleware>()
+		          .UseMiddleware<RequestVerificationMiddleware>()
 		          .UseMiddleware<RequestBufferingMiddleware>()
 		          .UseMiddleware<AuthenticationMiddleware>()
 		          .UseMiddleware<AuthorizationMiddleware>()

Iceshrimp.Backend/Core/Middleware/ErrorHandlerMiddleware.cs

@@ -25,6 +25,9 @@ public class ErrorHandlerMiddleware(IOptions<Config.SecuritySection> options, IL
 			var verbosity = options.Value.ExceptionVerbosity;
 
 			if (e is GracefulException ce) {
+				if (verbosity > ExceptionVerbosity.Basic && ce.OverrideBasic)
+					verbosity = ExceptionVerbosity.Basic;
+
 				ctx.Response.StatusCode = (int)ce.StatusCode;
 				await ctx.Response.WriteAsJsonAsync(new ErrorResponse {
 					StatusCode = ctx.Response.StatusCode,
@@ -58,10 +61,16 @@ public class ErrorHandlerMiddleware(IOptions<Config.SecuritySection> options, IL
 	}
 }
 
-public class GracefulException(HttpStatusCode statusCode, string error, string message, string? details = null)
+public class GracefulException(
+	HttpStatusCode statusCode,
+	string error,
+	string message,
+	string? details = null,
+	bool overrideBasic = false)
 	: Exception(message) {
 	public readonly string?        Details       = details;
 	public readonly string         Error         = error;
+	public readonly bool           OverrideBasic = overrideBasic;
 	public readonly HttpStatusCode StatusCode    = statusCode;
 
 	public GracefulException(HttpStatusCode statusCode, string message, string? details = null) :
@@ -93,6 +102,11 @@ public class GracefulException(HttpStatusCode statusCode, string error, string m
 	public static GracefulException RecordNotFound() {
 		return new GracefulException(HttpStatusCode.NotFound, "Record not found");
 	}
+
+	public static GracefulException MisdirectedRequest() {
+		return new GracefulException(HttpStatusCode.MisdirectedRequest, HttpStatusCode.MisdirectedRequest.ToString(),
+		                             "This server is not configured to respond to this request.", null, true);
+	}
 }
 
 public enum ExceptionVerbosity {

Iceshrimp.Backend/Core/Middleware/RequestVerificationMiddleware.cs

@@ -0,0 +1,27 @@
+using Iceshrimp.Backend.Core.Configuration;
+using Microsoft.Extensions.Options;
+
+namespace Iceshrimp.Backend.Core.Middleware;
+
+public class RequestVerificationMiddleware(IOptions<Config.InstanceSection> config, IHostEnvironment environment)
+	: IMiddleware {
+	private readonly bool _isDevelopment = environment.IsDevelopment();
+
+	public async Task InvokeAsync(HttpContext ctx, RequestDelegate next) {
+		if (!IsValid(ctx.Request) && !_isDevelopment)
+			throw GracefulException.MisdirectedRequest();
+
+		await next(ctx);
+	}
+
+	public bool IsValid(HttpRequest rq) {
+		if (rq.Host.Host == config.Value.WebDomain) return true;
+		if (rq.Host.Host == config.Value.AccountDomain && rq.Path.StartsWithSegments("/.well-known")) {
+			if (rq.Path == "/.well-known/webfinger") return true;
+			if (rq.Path == "/.well-known/host-meta") return true;
+			if (rq.Path == "/.well-known/nodeinfo") return true;
+		}
+
+		return false;
+	}
+}

Iceshrimp.Backend/Startup.cs

@@ -57,6 +57,6 @@ app.MapFallbackToPage("/Shared/FrontendSPA");
 if (app.Environment.IsDevelopment()) app.UseViteDevMiddleware();
 
 app.Urls.Clear();
-app.Urls.Add($"http://{config.WebDomain}:{config.ListenPort}");
+app.Urls.Add($"http://{config.ListenHost}:{config.ListenPort}");
 
 app.Run();

Iceshrimp.Backend/configuration.ini

@@ -1,5 +1,6 @@
 [Instance]
 ListenPort = 3000
+ListenHost = localhost
 
 ;; Caution: changing these settings after initial setup *will* break federation
 WebDomain = shrimp.example.org