allow ap/show to follow cross-domain redirects

1 files changed, 20 insertions, 1 deletions

diff --git a/packages/backend/src/server/api/endpoints/ap/show.ts b/packages/backend/src/server/api/endpoints/ap/show.ts
index 474ff81822..4904e3cb87 100644
--- a/packages/backend/src/server/api/endpoints/ap/show.ts
+++ b/packages/backend/src/server/api/endpoints/ap/show.ts
@@ -7,7 +7,7 @@ import { Inject, Injectable } from '@nestjs/common';
 import { Endpoint } from '@/server/api/endpoint-base.js';
 import type { MiNote } from '@/models/Note.js';
 import type { MiLocalUser, MiUser } from '@/models/User.js';
-import { isActor, isPost, getApId } from '@/core/activitypub/type.js';
+import { isActor, isPost, getApId, getNullableApId, ObjectWithId } from '@/core/activitypub/type.js';
 import type { SchemaType } from '@/misc/json-schema.js';
 import { ApResolverService } from '@/core/activitypub/ApResolverService.js';
 import { ApDbResolverService } from '@/core/activitypub/ApDbResolverService.js';
@@ -17,6 +17,8 @@ import { UserEntityService } from '@/core/entities/UserEntityService.js';
 import { NoteEntityService } from '@/core/entities/NoteEntityService.js';
 import { UtilityService } from '@/core/UtilityService.js';
 import { bindThis } from '@/decorators.js';
+import { ApRequestService } from '@/core/activitypub/ApRequestService.js';
+import { InstanceActorService } from '@/core/InstanceActorService.js';
 import { ApiError } from '../../error.js';
 
 export const meta = {
@@ -94,6 +96,8 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 		private apDbResolverService: ApDbResolverService,
 		private apPersonService: ApPersonService,
 		private apNoteService: ApNoteService,
+		private readonly apRequestService: ApRequestService,
+		private readonly instanceActorService: InstanceActorService,
 	) {
 		super(meta, paramDef, async (ps, me) => {
 			const object = await this.fetchAny(ps.uri, me);
@@ -118,6 +122,12 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 		]));
 		if (local != null) return local;
 
+		// No local object found with that uri.
+		// Before we fetch, resolve the URI in case it has a cross-origin redirect or anything like that.
+		// Resolver.resolve() uses strict verification, which is overly paranoid for a user-provided lookup.
+		uri = await this.resolveCanonicalUri(uri); // eslint-disable-line no-param-reassign
+		if (!this.utilityService.isFederationAllowedUri(uri)) return null;
+
 		const host = this.utilityService.extractDbHost(uri);
 
 		// local object, not found in db? fail
@@ -167,4 +177,13 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 
 		return null;
 	}
+
+	/**
+	 * Resolves an arbitrary URI to its canonical, post-redirect form.
+	 */
+	private async resolveCanonicalUri(uri: string): Promise<string> {
+		const user = await this.instanceActorService.getInstanceActor();
+		const res = await this.apRequestService.signedGet(uri, user, true) as ObjectWithId;
+		return getNullableApId(res) ?? uri;
+	}
 }