Iceshrimp.NET/Iceshrimp.Backend/Core/Middleware/InboxValidationMiddleware.cs

using System.Diagnostics.CodeAnalysis;
using System.Net;
using System.Net.Http.Headers;
using Iceshrimp.Backend.Core.Configuration;
using Iceshrimp.Backend.Core.Database;
using Iceshrimp.Backend.Core.Database.Tables;
using Iceshrimp.Backend.Core.Federation.ActivityStreams;
using Iceshrimp.Backend.Core.Federation.ActivityStreams.Types;
using Iceshrimp.Backend.Core.Federation.Cryptography;
using Iceshrimp.Backend.Core.Services;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Options;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;

namespace Iceshrimp.Backend.Core.Middleware;

public class InboxValidationMiddleware(
	[SuppressMessage("ReSharper", "SuggestBaseTypeForParameterInConstructor")]
	IOptionsSnapshot<Config.SecuritySection> config,
	DatabaseContext db,
	ActivityPub.UserResolver userResolver,
	UserService userSvc,
	ActivityPub.FederationControlService fedCtrlSvc,
	ILogger<InboxValidationMiddleware> logger,
	IHostApplicationLifetime appLifetime
) : IMiddleware
{
	private static readonly JsonSerializerSettings JsonSerializerSettings =
		new() { DateParseHandling = DateParseHandling.None };

	public async Task InvokeAsync(HttpContext ctx, RequestDelegate next)
	{
		var attribute = ctx.GetEndpoint()?.Metadata.GetMetadata<InboxValidationAttribute>();

		if (attribute != null)
		{
			var request = ctx.Request;
			var ct      = appLifetime.ApplicationStopping;

			if (request is not { ContentType: not null, ContentLength: > 0 })
				throw GracefulException.UnprocessableEntity("Inbox request must have a body");

			HttpSignature.HttpSignatureHeader? sig = null;

			if (request.Headers.TryGetValue("signature", out var sigHeader))
			{
				try
				{
					sig = HttpSignature.Parse(sigHeader.ToString());
					if (await fedCtrlSvc.ShouldBlockAsync(sig.KeyId))
						throw new GracefulException(HttpStatusCode.Forbidden, "Forbidden", "Instance is blocked",
						                            suppressLog: true);
				}
				catch (Exception e)
				{
					if (e is GracefulException { SuppressLog: true }) throw;
				}
			}

			var body = await new StreamReader(request.Body).ReadToEndAsync(ct);
			request.Body.Seek(0, SeekOrigin.Begin);
			var expanded = LdHelpers.Expand(JToken.Parse(body));
			if (expanded == null)
				throw new Exception("Failed to expand ASObject");
			var obj = ASObject.Deserialize(expanded);
			if (obj == null)
				throw new Exception("Failed to deserialize ASObject");
			if (obj is not ASActivity activity)
				throw new GracefulException("Request body is not an ASActivity", $"Type: {obj.Type}");

			UserPublickey? key      = null;
			var            verified = false;

			try
			{
				if (sig == null)
					throw new GracefulException(HttpStatusCode.Unauthorized, "Request is missing the signature header");

				// First, we check if we already have the key
				key = await db.UserPublickeys.Include(p => p.User)
				              .FirstOrDefaultAsync(p => p.KeyId == sig.KeyId, ct);

				// If we don't, we need to try to fetch it
				if (key == null)
				{
					try
					{
						var user = await userResolver.ResolveAsync(sig.KeyId, activity is ASDelete).WaitAsync(ct);
						if (user == null) throw AuthFetchException.NotFound("Delete activity actor is unknown");
						key = await db.UserPublickeys.Include(p => p.User)
						              .FirstOrDefaultAsync(p => p.User == user, ct);

						// If the key is still null here, we have a data consistency issue and need to update the key manually
						key ??= await userSvc.UpdateUserPublicKeyAsync(user).WaitAsync(ct);
					}
					catch (Exception e)
					{
						if (e is GracefulException) throw;
						throw new
							GracefulException($"Failed to fetch key of signature user ({sig.KeyId}) - {e.Message}");
					}
				}

				// If we still don't have the key, something went wrong and we need to throw an exception
				if (key == null) throw new GracefulException($"Failed to fetch key of signature user ({sig.KeyId})");

				if (key.User.Host == null)
					throw new GracefulException("Remote user must have a host");

				// We want to check both the user host & the keyId host (as account & web domain might be different)
				if (await fedCtrlSvc.ShouldBlockAsync(key.User.Host, key.KeyId))
					throw new GracefulException(HttpStatusCode.Forbidden, "Forbidden", "Instance is blocked",
					                            suppressLog: true);

				List<string> headers = ["(request-target)", "digest", "host"];

				if (sig.Created != null && !sig.Headers.Contains("date"))
					headers.Add("(created)");
				else
					headers.Add("date");

				verified = await HttpSignature.VerifyAsync(request, sig, headers, key.KeyPem);
				logger.LogDebug("HttpSignature.Verify returned {result} for key {keyId}", verified, sig.KeyId);

				if (!verified)
				{
					logger.LogDebug("Refetching user key...");
					key      = await userSvc.UpdateUserPublicKeyAsync(key);
					verified = await HttpSignature.VerifyAsync(request, sig, headers, key.KeyPem);
					logger.LogDebug("HttpSignature.Verify returned {result} for key {keyId}", verified, sig.KeyId);
				}
			}
			catch (Exception e)
			{
				if (e is AuthFetchException afe) throw GracefulException.Accepted(afe.Message);
				if (e is GracefulException { SuppressLog: true }) throw;
				logger.LogDebug("Error validating HTTP signature: {error}", e.Message);
			}

			if ((!verified || (key?.User.Uri != null && activity.Actor?.Id != key.User.Uri)) &&
			    (activity is ASDelete || config.Value.AcceptLdSignatures))
			{
				if (activity is ASDelete)
					logger.LogDebug("Activity is ASDelete & actor uri is not matching, trying LD signature next...");
				else
					logger.LogDebug("Trying LD signature next...");
				try
				{
					var contentType = new MediaTypeHeaderValue(request.ContentType);
					if (!ActivityPub.ActivityFetcherService.IsValidActivityContentType(contentType))
						throw new Exception("Request body is not an activity");

					if (activity.Actor == null)
						throw new Exception("Activity has no actor");
					if (await fedCtrlSvc.ShouldBlockAsync(new Uri(activity.Actor.Id).Host))
						throw new GracefulException(HttpStatusCode.Forbidden, "Forbidden", "Instance is blocked",
						                            suppressLog: true);
					key = null;
					key = await db.UserPublickeys
					              .Include(p => p.User)
					              .FirstOrDefaultAsync(p => p.User.Uri == activity.Actor.Id, ct);

					if (key == null)
					{
						var user = await userResolver.ResolveAsync(activity.Actor.Id, activity is ASDelete)
						                             .WaitAsync(ct);
						if (user == null) throw AuthFetchException.NotFound("Delete activity actor is unknown");
						key = await db.UserPublickeys
						              .Include(p => p.User)
						              .FirstOrDefaultAsync(p => p.User == user, ct);

						if (key == null)
							throw new Exception($"Failed to fetch public key for user {activity.Actor.Id}");
					}

					if (await fedCtrlSvc.ShouldBlockAsync(key.User.Host, new Uri(key.KeyId).Host))
						throw new GracefulException(HttpStatusCode.Forbidden, "Forbidden", "Instance is blocked",
						                            suppressLog: true);

					// We need to re-run deserialize & expand with date time handling disabled for JSON-LD canonicalization to work correctly
					var rawDeserialized = JsonConvert.DeserializeObject<JObject?>(body, JsonSerializerSettings);
					var rawExpanded     = LdHelpers.Expand(rawDeserialized);
					if (rawExpanded == null)
						throw new Exception("Failed to expand activity for LD signature processing");
					verified = await LdSignature.VerifyAsync(expanded, rawExpanded, key.KeyPem, key.KeyId);
					logger.LogDebug("LdSignature.VerifyAsync returned {result} for actor {id}",
					                verified, activity.Actor.Id);
					if (!verified)
					{
						logger.LogDebug("Refetching user key...");
						key      = await userSvc.UpdateUserPublicKeyAsync(key);
						verified = await LdSignature.VerifyAsync(expanded, rawExpanded, key.KeyPem, key.KeyId);
						logger.LogDebug("LdSignature.VerifyAsync returned {result} for actor {id}",
						                verified, activity.Actor.Id);
					}
				}
				catch (Exception e)
				{
					if (e is AuthFetchException afe) throw GracefulException.Accepted(afe.Message);
					if (e is GracefulException { SuppressLog: true }) throw;
					logger.LogError("Error validating JSON-LD signature: {error}", e.Message);
				}
			}

			if (!verified || key == null)
				throw new GracefulException(HttpStatusCode.Forbidden, "Request signature validation failed");

			ctx.SetActor(key.User);
		}

		await next(ctx);
	}
}

public class InboxValidationAttribute : Attribute;