diff --git a/packages/backend/src/core/IdService.ts b/packages/backend/src/core/IdService.ts
index c98b8ea6fc..43e72d2d7b 100644
--- a/packages/backend/src/core/IdService.ts
+++ b/packages/backend/src/core/IdService.ts
@@ -7,11 +7,11 @@ import { Inject, Injectable } from '@nestjs/common';
 import { ulid } from 'ulid';
 import { DI } from '@/di-symbols.js';
 import type { Config } from '@/config.js';
-import { genAid, parseAid } from '@/misc/id/aid.js';
-import { genAidx, parseAidx } from '@/misc/id/aidx.js';
-import { genMeid, parseMeid } from '@/misc/id/meid.js';
-import { genMeidg, parseMeidg } from '@/misc/id/meidg.js';
-import { genObjectId, parseObjectId } from '@/misc/id/object-id.js';
+import { genAid, isSafeAidT, parseAid } from '@/misc/id/aid.js';
+import { genAidx, isSafeAidxT, parseAidx } from '@/misc/id/aidx.js';
+import { genMeid, isSafeMeidT, parseMeid } from '@/misc/id/meid.js';
+import { genMeidg, isSafeMeidgT, parseMeidg } from '@/misc/id/meidg.js';
+import { genObjectId, isSafeObjectIdT, parseObjectId } from '@/misc/id/object-id.js';
 import { bindThis } from '@/decorators.js';
 import { parseUlid } from '@/misc/id/ulid.js';
 
@@ -26,6 +26,19 @@ export class IdService {
 		this.method = config.id.toLowerCase();
 	}
 
+	@bindThis
+	public isSafeT(t: number): boolean {
+		switch (this.method) {
+			case 'aid': return isSafeAidT(t);
+			case 'aidx': return isSafeAidxT(t);
+			case 'meid': return isSafeMeidT(t);
+			case 'meidg': return isSafeMeidgT(t);
+			case 'ulid': return t > 0;
+			case 'objectid': return isSafeObjectIdT(t);
+			default: throw new Error('unrecognized id generation method');
+		}
+	}
+
 	/**
 	 * 時間を元にIDを生成します(省略時は現在日時)
 	 * @param time 日時
diff --git a/packages/backend/src/core/activitypub/ApInboxService.ts b/packages/backend/src/core/activitypub/ApInboxService.ts
index 7aba140689..baaab67e48 100644
--- a/packages/backend/src/core/activitypub/ApInboxService.ts
+++ b/packages/backend/src/core/activitypub/ApInboxService.ts
@@ -306,9 +306,15 @@ export class ApInboxService {
 			this.logger.info(`Creating the (Re)Note: ${uri}`);
 
 			const activityAudience = await this.apAudienceService.parseAudience(actor, activity.to, activity.cc);
+			const createdAt = activity.published ? new Date(activity.published) : null;
+
+			if (createdAt && createdAt < this.idService.parse(renote.id).date) {
+				this.logger.warn('skip: malformed createdAt');
+				return;
+			}
 
 			await this.noteCreateService.create(actor, {
-				createdAt: activity.published ? new Date(activity.published) : null,
+				createdAt,
 				renote,
 				visibility: activityAudience.visibility,
 				visibleUsers: activityAudience.visibleUsers,
diff --git a/packages/backend/src/core/activitypub/models/ApNoteService.ts b/packages/backend/src/core/activitypub/models/ApNoteService.ts
index 1979cdda9c..05d5ca15db 100644
--- a/packages/backend/src/core/activitypub/models/ApNoteService.ts
+++ b/packages/backend/src/core/activitypub/models/ApNoteService.ts
@@ -92,6 +92,10 @@ export class ApNoteService {
 			return new Error(`invalid Note: attributedTo has different host. expected: ${expectHost}, actual: ${actualHost}`);
 		}
 
+		if (object.published && !this.idService.isSafeT(new Date(object.published).valueOf())) {
+			return new Error('invalid Note: published timestamp is malformed');
+		}
+
 		return null;
 	}
 
diff --git a/packages/backend/src/misc/id/aid.ts b/packages/backend/src/misc/id/aid.ts
index e7b59f262b..de03f6793f 100644
--- a/packages/backend/src/misc/id/aid.ts
+++ b/packages/backend/src/misc/id/aid.ts
@@ -34,3 +34,7 @@ export function parseAid(id: string): { date: Date; } {
 	const time = parseInt(id.slice(0, 8), 36) + TIME2000;
 	return { date: new Date(time) };
 }
+
+export function isSafeAidT(t: number): boolean {
+	return t > TIME2000;
+}
diff --git a/packages/backend/src/misc/id/aidx.ts b/packages/backend/src/misc/id/aidx.ts
index bed223225a..9f457f6f0a 100644
--- a/packages/backend/src/misc/id/aidx.ts
+++ b/packages/backend/src/misc/id/aidx.ts
@@ -41,3 +41,7 @@ export function parseAidx(id: string): { date: Date; } {
 	const time = parseInt(id.slice(0, TIME_LENGTH), 36) + TIME2000;
 	return { date: new Date(time) };
 }
+
+export function isSafeAidxT(t: number): boolean {
+	return t > TIME2000;
+}
diff --git a/packages/backend/src/misc/id/meid.ts b/packages/backend/src/misc/id/meid.ts
index 366738de05..7646282edb 100644
--- a/packages/backend/src/misc/id/meid.ts
+++ b/packages/backend/src/misc/id/meid.ts
@@ -38,3 +38,7 @@ export function parseMeid(id: string): { date: Date; } {
 		date: new Date(parseInt(id.slice(0, 12), 16) - 0x800000000000),
 	};
 }
+
+export function isSafeMeidT(t: number): boolean {
+	return t > 0;
+}
diff --git a/packages/backend/src/misc/id/meidg.ts b/packages/backend/src/misc/id/meidg.ts
index 426a46970b..f2a55443ef 100644
--- a/packages/backend/src/misc/id/meidg.ts
+++ b/packages/backend/src/misc/id/meidg.ts
@@ -38,3 +38,7 @@ export function parseMeidg(id: string): { date: Date; } {
 		date: new Date(parseInt(id.slice(1, 12), 16)),
 	};
 }
+
+export function isSafeMeidgT(t: number): boolean {
+	return t > 0;
+}
diff --git a/packages/backend/src/misc/id/object-id.ts b/packages/backend/src/misc/id/object-id.ts
index 49bd9591c0..f5c3619fdb 100644
--- a/packages/backend/src/misc/id/object-id.ts
+++ b/packages/backend/src/misc/id/object-id.ts
@@ -38,3 +38,7 @@ export function parseObjectId(id: string): { date: Date; } {
 		date: new Date(parseInt(id.slice(0, 8), 16) * 1000),
 	};
 }
+
+export function isSafeObjectIdT(t: number): boolean {
+	return t > 0;
+}