diff --git a/packages/backend/src/models/entities/instance.ts b/packages/backend/src/models/entities/instance.ts
index ab7b2fa456..afdedd2d3f 100644
--- a/packages/backend/src/models/entities/instance.ts
+++ b/packages/backend/src/models/entities/instance.ts
@@ -1,6 +1,19 @@
 import { Entity, PrimaryColumn, Index, Column } from "typeorm";
 import { id } from "../id.js";
 
+export const MAX_LENGTH_INSTANCE = {
+	host: 512,
+	softwareName: 64,
+	softwareVersion: 64,
+	name: 256,
+	description: 4096,
+	maintainerName: 128,
+	maintainerEmail: 256,
+	iconUrl: 4096,
+	faviconUrl: 4096,
+	themeColor: 64,
+};
+
 @Entity()
 export class Instance {
 	@PrimaryColumn(id())
@@ -20,7 +33,7 @@ export class Instance {
 	 */
 	@Index({ unique: true })
 	@Column("varchar", {
-		length: 512,
+		length: MAX_LENGTH_INSTANCE.host,
 		comment: "The host of the Instance.",
 	})
 	public host: string;
@@ -107,14 +120,14 @@ export class Instance {
 	public isSuspended: boolean;
 
 	@Column("varchar", {
-		length: 64,
+		length: MAX_LENGTH_INSTANCE.softwareName,
 		nullable: true,
 		comment: "The software of the Instance.",
 	})
 	public softwareName: string | null;
 
 	@Column("varchar", {
-		length: 64,
+		length: MAX_LENGTH_INSTANCE.softwareVersion,
 		nullable: true,
 	})
 	public softwareVersion: string | null;
@@ -125,43 +138,43 @@ export class Instance {
 	public openRegistrations: boolean | null;
 
 	@Column("varchar", {
-		length: 256,
+		length: MAX_LENGTH_INSTANCE.name,
 		nullable: true,
 	})
 	public name: string | null;
 
 	@Column("varchar", {
-		length: 4096,
+		length: MAX_LENGTH_INSTANCE.description,
 		nullable: true,
 	})
 	public description: string | null;
 
 	@Column("varchar", {
-		length: 128,
+		length: MAX_LENGTH_INSTANCE.maintainerName,
 		nullable: true,
 	})
 	public maintainerName: string | null;
 
 	@Column("varchar", {
-		length: 256,
+		length: MAX_LENGTH_INSTANCE.maintainerEmail,
 		nullable: true,
 	})
 	public maintainerEmail: string | null;
 
 	@Column("varchar", {
-		length: 4096,
+		length: MAX_LENGTH_INSTANCE.iconUrl,
 		nullable: true,
 	})
 	public iconUrl: string | null;
 
 	@Column("varchar", {
-		length: 4096,
+		length: MAX_LENGTH_INSTANCE.faviconUrl,
 		nullable: true,
 	})
 	public faviconUrl: string | null;
 
 	@Column("varchar", {
-		length: 64,
+		length: MAX_LENGTH_INSTANCE.themeColor,
 		nullable: true,
 	})
 	public themeColor: string | null;
diff --git a/packages/backend/src/services/fetch-instance-metadata.ts b/packages/backend/src/services/fetch-instance-metadata.ts
index 6795db0690..18e6411e88 100644
--- a/packages/backend/src/services/fetch-instance-metadata.ts
+++ b/packages/backend/src/services/fetch-instance-metadata.ts
@@ -3,7 +3,10 @@ import { Window } from "happy-dom";
 import fetch from "node-fetch";
 import tinycolor from "tinycolor2";
 import { getJson, getAgentByUrl } from "@/misc/fetch.js";
-import type { Instance } from "@/models/entities/instance.js";
+import {
+	type Instance,
+	MAX_LENGTH_INSTANCE,
+} from "@/models/entities/instance.js";
 import { Instances } from "@/models/index.js";
 import { getFetchInstanceMetadataLock } from "@/misc/app-lock.js";
 import Logger from "@/services/logger.js";
@@ -53,26 +56,52 @@ export async function fetchInstanceMetadata(
 		} as Record<string, any>;
 
 		if (info) {
-			updates.softwareName = info.software?.name?.toLowerCase() || null;
-			updates.softwareVersion = info.software?.version;
+			updates.softwareName =
+				info.software?.name
+					?.toLowerCase()
+					.substring(0, MAX_LENGTH_INSTANCE.softwareName) || null;
+			updates.softwareVersion =
+				info.software?.version?.substring(
+					0,
+					MAX_LENGTH_INSTANCE.softwareVersion,
+				) || null;
 			updates.openRegistrations = info.openRegistrations;
 			updates.maintainerName = info.metadata
 				? info.metadata.maintainer
-					? info.metadata.maintainer.name || null
+					? info.metadata.maintainer.name?.substring(
+							0,
+							MAX_LENGTH_INSTANCE.maintainerName,
+						) || null
 					: null
 				: null;
 			updates.maintainerEmail = info.metadata
 				? info.metadata.maintainer
-					? info.metadata.maintainer.email || null
+					? info.metadata.maintainer.email?.substring(
+							0,
+							MAX_LENGTH_INSTANCE.maintainerEmail,
+						) || null
 					: null
 				: null;
 		}
 
-		if (name) updates.name = name;
-		if (description) updates.description = description;
-		if (icon || favicon) updates.iconUrl = icon || favicon;
-		if (favicon) updates.faviconUrl = favicon;
-		if (themeColor) updates.themeColor = themeColor;
+		if (name) updates.name = name.substring(0, MAX_LENGTH_INSTANCE.name);
+		if (description)
+			updates.description = description.substring(
+				0,
+				MAX_LENGTH_INSTANCE.description,
+			);
+		if (icon || favicon)
+			updates.iconUrl = (icon || favicon)?.substring(
+				0,
+				MAX_LENGTH_INSTANCE.iconUrl,
+			);
+		if (favicon)
+			updates.faviconUrl = favicon.substring(0, MAX_LENGTH_INSTANCE.faviconUrl);
+		if (themeColor)
+			updates.themeColor = themeColor.substring(
+				0,
+				MAX_LENGTH_INSTANCE.themeColor,
+			);
 
 		await Instances.update(instance.id, updates);
 
diff --git a/packages/backend/src/services/register-or-fetch-instance-doc.ts b/packages/backend/src/services/register-or-fetch-instance-doc.ts
index c0ead08190..be26d8689b 100644
--- a/packages/backend/src/services/register-or-fetch-instance-doc.ts
+++ b/packages/backend/src/services/register-or-fetch-instance-doc.ts
@@ -1,9 +1,14 @@
-import type { Instance } from "@/models/entities/instance.js";
+import {
+	type Instance,
+	MAX_LENGTH_INSTANCE,
+} from "@/models/entities/instance.js";
 import { Instances } from "@/models/index.js";
 import { genId } from "@/misc/gen-id.js";
 import { toPuny } from "@/misc/convert-host.js";
 import { Cache } from "@/misc/cache.js";
+import Logger from "@/services/logger.js";
 
+const logger = new Logger("register-or-fetch-instance");
 const cache = new Cache<Instance>("registerOrFetchInstanceDoc", 60 * 60);
 
 export async function registerOrFetchInstanceDoc(
@@ -11,6 +16,14 @@ export async function registerOrFetchInstanceDoc(
 ): Promise<Instance> {
 	const _host = toPuny(host);
 
+	if (_host.length > MAX_LENGTH_INSTANCE.host) {
+		logger.error(
+			`Instance host name must not be longer than ${MAX_LENGTH_INSTANCE.host} characters`,
+		);
+		logger.error(`hostname: ${_host}`);
+		throw new Error("Instance host name is too long");
+	}
+
 	const cached = await cache.get(_host);
 	if (cached) return cached;