diff --git a/packages/backend/package.json b/packages/backend/package.json
index ca9a5f0f90..388a3d3c7a 100644
--- a/packages/backend/package.json
+++ b/packages/backend/package.json
@@ -60,7 +60,6 @@
 	"dependencies": {
 		"@aws-sdk/client-s3": "3.412.0",
 		"@aws-sdk/lib-storage": "3.412.0",
-		"@smithy/node-http-handler": "2.1.5",
 		"@bull-board/api": "5.9.1",
 		"@bull-board/fastify": "5.9.1",
 		"@bull-board/ui": "5.9.1",
@@ -81,6 +80,7 @@
 		"@peertube/http-signature": "1.7.0",
 		"@simplewebauthn/server": "8.3.5",
 		"@sinonjs/fake-timers": "11.2.2",
+		"@smithy/node-http-handler": "2.1.5",
 		"@swc/cli": "0.1.62",
 		"@swc/core": "1.3.95",
 		"@vitalets/google-translate-api": "9.2.0",
@@ -105,6 +105,7 @@
 		"date-fns": "2.30.0",
 		"deep-email-validator": "0.1.21",
 		"fastify": "4.24.3",
+		"fastify-raw-body": "^4.2.2",
 		"feed": "4.2.2",
 		"file-type": "18.6.0",
 		"fluent-ffmpeg": "2.1.2",
diff --git a/packages/backend/src/server/ActivityPubServerService.ts b/packages/backend/src/server/ActivityPubServerService.ts
index cfc8576435..fa44ce0170 100644
--- a/packages/backend/src/server/ActivityPubServerService.ts
+++ b/packages/backend/src/server/ActivityPubServerService.ts
@@ -3,6 +3,7 @@
  * SPDX-License-Identifier: AGPL-3.0-only
  */
 
+import * as crypto from 'node:crypto';
 import { IncomingMessage } from 'node:http';
 import { Inject, Injectable } from '@nestjs/common';
 import fastifyAccepts from '@fastify/accepts';
@@ -108,7 +109,58 @@ export class ActivityPubServerService {
 			return;
 		}
 
-		// TODO: request.bodyのバリデーション？
+		if (signature.params.headers.indexOf('host') === -1
+			|| request.headers.host !== this.config.host) {
+			// Host not specified or not match.
+			reply.code(401);
+			return;
+		}
+
+		if (signature.params.headers.indexOf('digest') === -1) {
+			// Digest not found.
+			reply.code(401);
+		} else {
+			const digest = request.headers.digest;
+
+			if (typeof digest !== 'string') {
+				// Huh?
+				reply.code(401);
+				return;
+			}
+
+			const re = /^([a-zA-Z0-9\-]+)=(.+)$/;
+			const match = digest.match(re);
+
+			if (match == null) {
+				// Invalid digest
+				reply.code(401);
+				return;
+			}
+
+			const algo = match[1];
+			const digestValue = match[2];
+
+			if (algo !== 'SHA-256') {
+				// Unsupported digest algorithm
+				reply.code(401);
+				return;
+			}
+
+			if (request.rawBody == null) {
+				// Bad request
+				reply.code(400);
+				return;
+			}
+
+			const hash = crypto.createHash('sha256').update(request.rawBody).digest('base64');
+
+			if (hash !== digestValue) {
+				// Invalid digest
+				reply.code(401);
+				return;
+			}
+		}
+
 		this.queueService.inbox(request.body as IActivity, signature);
 
 		reply.code(202);
@@ -474,8 +526,8 @@ export class ActivityPubServerService {
 
 		//#region Routing
 		// inbox (limit: 64kb)
-		fastify.post('/inbox', { bodyLimit: 1024 * 64 }, async (request, reply) => await this.inbox(request, reply));
-		fastify.post('/users/:user/inbox', { bodyLimit: 1024 * 64 }, async (request, reply) => await this.inbox(request, reply));
+		fastify.post('/inbox', { config: { rawBody: true }, bodyLimit: 1024 * 64 }, async (request, reply) => await this.inbox(request, reply));
+		fastify.post('/users/:user/inbox', { config: { rawBody: true }, bodyLimit: 1024 * 64 }, async (request, reply) => await this.inbox(request, reply));
 
 		// note
 		fastify.get<{ Params: { note: string; } }>('/notes/:note', { constraints: { apOrHtml: 'ap' } }, async (request, reply) => {
diff --git a/packages/backend/src/server/ServerService.ts b/packages/backend/src/server/ServerService.ts
index 4b4b91b402..e411184bc1 100644
--- a/packages/backend/src/server/ServerService.ts
+++ b/packages/backend/src/server/ServerService.ts
@@ -9,6 +9,7 @@ import { fileURLToPath } from 'node:url';
 import { Inject, Injectable, OnApplicationShutdown } from '@nestjs/common';
 import Fastify, { FastifyInstance } from 'fastify';
 import fastifyStatic from '@fastify/static';
+import fastifyRawBody from 'fastify-raw-body';
 import { IsNull } from 'typeorm';
 import { GlobalEventService } from '@/core/GlobalEventService.js';
 import type { Config } from '@/config.js';
@@ -86,6 +87,13 @@ export class ServerService implements OnApplicationShutdown {
 			});
 		}
 
+		// Register raw-body parser for ActivityPub HTTP signature validation.
+		fastify.register(fastifyRawBody, {
+			global: false,
+			encoding: 'utf-8',
+			runFirst: true,
+		});
+
 		// Register non-serving static server so that the child services can use reply.sendFile.
 		// `root` here is just a placeholder and each call must use its own `rootPath`.
 		fastify.register(fastifyStatic, {
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index b11dd62d37..34a8e94ea9 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -1,4 +1,4 @@
-lockfileVersion: '6.0'
+lockfileVersion: '6.1'
 
 settings:
   autoInstallPeers: true
@@ -197,6 +197,9 @@ importers:
       fastify:
         specifier: 4.24.3
         version: 4.24.3
+      fastify-raw-body:
+        specifier: ^4.2.2
+        version: 4.2.2
       feed:
         specifier: 4.2.2
         version: 4.2.2
@@ -7169,7 +7172,7 @@ packages:
     hasBin: true
     peerDependencies:
       '@swc/core': ^1.2.66
-      chokidar: 3.5.3
+      chokidar: ^3.5.1
     peerDependenciesMeta:
       chokidar:
         optional: true
@@ -12058,6 +12061,15 @@ packages:
     resolution: {integrity: sha512-79ak0JxddO0utAXAQ5ccKhvs6vX2MGyHHMMsmZkBANrq3hXc1CHzvNPHOcvTsVMEPl5I+NT+RO4YKMGehOfSIg==}
     dev: false
 
+  /fastify-raw-body@4.2.2:
+    resolution: {integrity: sha512-6l4fXtxNn7WOQiylu5fv9/JfUTvWCg1ED4gF44hqnVesgttOXEUMnNkdV8ZxwufCstRyUYaYSBIN4VuRHDbJkw==}
+    engines: {node: '>= 10'}
+    dependencies:
+      fastify-plugin: 4.5.0
+      raw-body: 2.5.2
+      secure-json-parse: 2.7.0
+    dev: false
+
   /fastify@4.24.3:
     resolution: {integrity: sha512-6HHJ+R2x2LS3y1PqxnwEIjOTZxFl+8h4kSC/TuDPXtA+v2JnV9yEtOsNSKK1RMD7sIR2y1ZsA4BEFaid/cK5pg==}
     dependencies: