diff --git a/packages/backend/src/queue/processors/inbox.ts b/packages/backend/src/queue/processors/inbox.ts
index c8a0920316..6fb5376520 100644
--- a/packages/backend/src/queue/processors/inbox.ts
+++ b/packages/backend/src/queue/processors/inbox.ts
@@ -22,6 +22,7 @@ import { StatusError } from "@/misc/fetch.js";
 import type { CacheableRemoteUser } from "@/models/entities/user.js";
 import type { UserPublickey } from "@/models/entities/user-publickey.js";
 import { shouldBlockInstance } from "@/misc/should-block-instance.js";
+import { verifySignature } from "@/remote/activitypub/check-fetch.js";
 
 const logger = new Logger("inbox");
 
@@ -114,6 +115,10 @@ export default async (job: Bull.Job<InboxJobData>): Promise<string> => {
 		);
 	}
 
+	if (httpSignatureValidated) {
+		if (!verifySignature(signature, authUser.key)) return `skip: Invalid HTTP signature`;
+	}
+
 	// また、signatureのsignerは、activity.actorと一致する必要がある
 	if (!httpSignatureValidated || authUser.user.uri !== activity.actor) {
 		// 一致しなくても、でもLD-Signatureがありそうならそっちも見る
diff --git a/packages/backend/src/remote/activitypub/check-fetch.ts b/packages/backend/src/remote/activitypub/check-fetch.ts
index b7fa179ac8..2887d13e8b 100644
--- a/packages/backend/src/remote/activitypub/check-fetch.ts
+++ b/packages/backend/src/remote/activitypub/check-fetch.ts
@@ -1,5 +1,5 @@
 import { URL } from "url";
-import httpSignature from "@peertube/http-signature";
+import httpSignature, { IParsedSignature } from "@peertube/http-signature";
 import config from "@/config/index.js";
 import { fetchMeta } from "@/misc/fetch-meta.js";
 import { toPuny } from "@/misc/convert-host.js";
@@ -9,6 +9,9 @@ import { shouldBlockInstance } from "@/misc/should-block-instance.js";
 import type { IncomingMessage } from "http";
 import type { CacheableRemoteUser } from "@/models/entities/user.js";
 import type { UserPublickey } from "@/models/entities/user-publickey.js";
+import { verify } from "node:crypto";
+import { toSingle } from "@/prelude/array.js";
+import { createHash } from "node:crypto";
 
 export async function hasSignature(req: IncomingMessage): Promise<string> {
 	const meta = await fetchMeta();
@@ -28,10 +31,12 @@ export async function hasSignature(req: IncomingMessage): Promise<string> {
 export async function checkFetch(req: IncomingMessage): Promise<number> {
 	const meta = await fetchMeta();
 	if (meta.secureMode || meta.privateMode) {
+		if (req.headers.host !== config.host) return 400;
+
 		let signature;
 
 		try {
-			signature = httpSignature.parseRequest(req, { headers: [] });
+			signature = httpSignature.parseRequest(req, { headers: ["(request-target)", "host", "date"] });
 		} catch (e) {
 			return 401;
 		}
@@ -108,6 +113,8 @@ export async function checkFetch(req: IncomingMessage): Promise<number> {
 		if (!httpSignatureValidated) {
 			return 403;
 		}
+
+		return verifySignature(signature, authUser.key) ? 200 : 401;
 	}
 	return 200;
 }
@@ -130,3 +137,22 @@ export async function getSignatureUser(req: IncomingMessage): Promise<{
 	keyId.hash = "";
 	return await dbResolver.getAuthUserFromApId(getApId(keyId.toString()));
 }
+
+export function verifySignature(sig: IParsedSignature, key: UserPublickey): boolean {
+	if (!['hs2019', 'rsa-sha256'].includes(sig.algorithm.toLowerCase())) return false;
+	try {
+		return verify('rsa-sha256', Buffer.from(sig.signingString, 'utf8'), key.keyPem, Buffer.from(sig.params.signature, 'base64'));
+	}
+	catch {
+		// Algo not supported
+		return false;
+	}
+}
+
+export function verifyDigest(body: string, digest: string | string[] | undefined): boolean {
+	digest = toSingle(digest);
+	if (body == null || digest == null || !digest.toLowerCase().startsWith('sha-256='))
+		return false;
+
+	return createHash('sha256').update(body).digest('base64') === digest.substring(8);
+}
diff --git a/packages/backend/src/server/activitypub.ts b/packages/backend/src/server/activitypub.ts
index 337db59933..2d61ae0d35 100644
--- a/packages/backend/src/server/activitypub.ts
+++ b/packages/backend/src/server/activitypub.ts
@@ -1,5 +1,5 @@
 import Router from "@koa/router";
-import json from "koa-json-body";
+import bodyParser from "koa-bodyparser";
 import httpSignature from "@peertube/http-signature";
 
 import { In, IsNull, Not } from "typeorm";
@@ -23,6 +23,7 @@ import { getUserKeypair } from "@/misc/keypair-store.js";
 import {
 	checkFetch,
 	getSignatureUser,
+	verifyDigest,
 } from "@/remote/activitypub/check-fetch.js";
 import { getInstanceActor } from "@/services/instance-actor.js";
 import { fetchMeta } from "@/misc/fetch-meta.js";
@@ -32,6 +33,8 @@ import Following from "./activitypub/following.js";
 import Followers from "./activitypub/followers.js";
 import Outbox, { packActivity } from "./activitypub/outbox.js";
 import { serverLogger } from "./index.js";
+import config from "@/config/index.js";
+import Koa from "koa";
 
 // Init router
 const router = new Router();
@@ -39,15 +42,25 @@ const router = new Router();
 //#region Routing
 
 function inbox(ctx: Router.RouterContext) {
+	if (ctx.req.headers.host !== config.host) {
+		ctx.status = 400;
+		return;
+	}
+
 	let signature;
 
 	try {
-		signature = httpSignature.parseRequest(ctx.req, { headers: [] });
+		signature = httpSignature.parseRequest(ctx.req, { headers: ['(request-target)', 'digest', 'host', 'date'] });
 	} catch (e) {
 		ctx.status = 401;
 		return;
 	}
 
+	if (!verifyDigest(ctx.request.rawBody, ctx.headers.digest)) {
+		ctx.status = 401;
+		return;
+	}
+
 	processInbox(ctx.request.body, signature);
 
 	ctx.status = 202;
@@ -72,9 +85,24 @@ export function setResponseType(ctx: Router.RouterContext) {
 	}
 }
 
+async function parseJsonBodyOrFail(ctx: Router.RouterContext, next: Koa.Next) {
+	const koaBodyParser = bodyParser({
+		enableTypes: ["json"],
+		detectJSON: () => true,
+	});
+
+	try {
+		await koaBodyParser(ctx, next);
+	}
+	catch {
+		ctx.status = 400;
+		return;
+	}
+}
+
 // inbox
-router.post("/inbox", json(), inbox);
-router.post("/users/:user/inbox", json(), inbox);
+router.post("/inbox", parseJsonBodyOrFail, inbox);
+router.post("/users/:user/inbox", parseJsonBodyOrFail, inbox);
 
 // note
 router.get("/notes/:note", async (ctx, next) => {