fix(server): "forkbomb" DOS mitigation (#9247)

* Add recursion limit to resolver

* Use shared resolver in featured and question

* Changelog

* Changelog fix

* Update CHANGELOG.md

Co-authored-by: Acid Chicken (硫酸鶏) <root@acid-chicken.com>

* Add host to recursion limit error message

Co-authored-by: Acid Chicken (硫酸鶏) <root@acid-chicken.com>

Co-authored-by: syuilo <Syuilotan@yahoo.co.jp>
Co-authored-by: Acid Chicken (硫酸鶏) <root@acid-chicken.com>

4 files changed, 12 insertions, 7 deletions

diff --git a/packages/backend/src/core/remote/activitypub/ApInboxService.ts b/packages/backend/src/core/remote/activitypub/ApInboxService.ts
index baeeb1ad3e..3da384ec2d 100644
--- a/packages/backend/src/core/remote/activitypub/ApInboxService.ts
+++ b/packages/backend/src/core/remote/activitypub/ApInboxService.ts
@@ -731,7 +731,7 @@ export class ApInboxService {
 			await this.apPersonService.updatePerson(actor.uri!, resolver, object);
 			return 'ok: Person updated';
 		} else if (getApType(object) === 'Question') {
-			await this.apQuestionService.updateQuestion(object).catch(err => console.error(err));
+			await this.apQuestionService.updateQuestion(object, resolver).catch(err => console.error(err));
 			return 'ok: Question updated';
 		} else {
 			return `skip: Unknown type: ${getApType(object)}`;
diff --git a/packages/backend/src/core/remote/activitypub/ApResolverService.ts b/packages/backend/src/core/remote/activitypub/ApResolverService.ts
index fe57c82ccc..bcdb9383d1 100644
--- a/packages/backend/src/core/remote/activitypub/ApResolverService.ts
+++ b/packages/backend/src/core/remote/activitypub/ApResolverService.ts
@@ -76,6 +76,7 @@ export class Resolver {
 		private httpRequestService: HttpRequestService,
 		private apRendererService: ApRendererService,
 		private apDbResolverService: ApDbResolverService,
+		private recursionLimit = 100
 	) {
 		this.history = new Set();
 	}
@@ -116,6 +117,10 @@ export class Resolver {
 			throw new Error('cannot resolve already resolved one');
 		}
 
+		if (this.history.size > this.recursionLimit) {
+			throw new Error(`hit recursion limit: ${this.utilityService.extractDbHost(value)}`);
+		}
+
 		this.history.add(value);
 
 		const host = this.utilityService.extractDbHost(value);
diff --git a/packages/backend/src/core/remote/activitypub/models/ApPersonService.ts b/packages/backend/src/core/remote/activitypub/models/ApPersonService.ts
index 5135473862..f9d6f42ef6 100644
--- a/packages/backend/src/core/remote/activitypub/models/ApPersonService.ts
+++ b/packages/backend/src/core/remote/activitypub/models/ApPersonService.ts
@@ -390,7 +390,7 @@ export class ApPersonService implements OnModuleInit {
 	});
 	//#endregion
 
-	await this.updateFeatured(user!.id).catch(err => this.logger.error(err));
+	await this.updateFeatured(user!.id, resolver).catch(err => this.logger.error(err));
 
 	return user!;
 	}
@@ -503,7 +503,7 @@ export class ApPersonService implements OnModuleInit {
 			followerSharedInbox: person.sharedInbox ?? (person.endpoints ? person.endpoints.sharedInbox : undefined),
 		});
 
-		await this.updateFeatured(exist.id).catch(err => this.logger.error(err));
+		await this.updateFeatured(exist.id, resolver).catch(err => this.logger.error(err));
 	}
 
 	/**
@@ -551,14 +551,14 @@ export class ApPersonService implements OnModuleInit {
 		return { fields, services };
 	}
 
-	public async updateFeatured(userId: User['id']) {
+	public async updateFeatured(userId: User['id'], resolver?: Resolver) {
 		const user = await this.usersRepository.findOneByOrFail({ id: userId });
 		if (!this.userEntityService.isRemoteUser(user)) return;
 		if (!user.featured) return;
 
 		this.logger.info(`Updating the featured: ${user.uri}`);
 
-		const resolver = this.apResolverService.createResolver();
+		if (resolver == null) resolver = this.apResolverService.createResolver();
 
 		// Resolve to (Ordered)Collection Object
 		const collection = await resolver.resolveCollection(user.featured);
diff --git a/packages/backend/src/core/remote/activitypub/models/ApQuestionService.ts b/packages/backend/src/core/remote/activitypub/models/ApQuestionService.ts
index acd5cdae83..5793b98353 100644
--- a/packages/backend/src/core/remote/activitypub/models/ApQuestionService.ts
+++ b/packages/backend/src/core/remote/activitypub/models/ApQuestionService.ts
@@ -65,7 +65,7 @@ export class ApQuestionService {
 	 * @param uri URI of AP Question object
 	 * @returns true if updated
 	 */
-	public async updateQuestion(value: any) {
+	public async updateQuestion(value: any, resolver?: Resolver) {
 		const uri = typeof value === 'string' ? value : value.id;
 
 		// URIがこのサーバーを指しているならスキップ
@@ -80,7 +80,7 @@ export class ApQuestionService {
 		//#endregion
 
 		// resolve new Question object
-		const resolver = this.apResolverService.createResolver();
+		if (resolver == null) resolver = this.apResolverService.createResolver();
 		const question = await resolver.resolve(value) as IQuestion;
 		this.logger.debug(`fetched question: ${JSON.stringify(question, null, 2)}`);