backend: check against url

packages/backend/src/core/HttpRequestService.ts

@@ -15,6 +15,7 @@ import type { Config } from '@/config.js';
 import { StatusError } from '@/misc/status-error.js';
 import { bindThis } from '@/decorators.js';
 import { validateContentTypeSetAsActivityPub } from '@/core/activitypub/misc/validator.js';
+import { assertActivityMatchesUrls } from '@/core/activitypub/misc/check-against-url.js';
 import type { IObject } from '@/core/activitypub/type.js';
 import type { Response } from 'node-fetch';
 import type { URL } from 'node:url';
@@ -125,7 +126,12 @@ export class HttpRequestService {
 			validators: [validateContentTypeSetAsActivityPub],
 		});
 
-		return await res.json() as IObject;
+		const finalUrl = res.url; // redirects may have been involved
+		const activity = await res.json() as IObject;
+
+		assertActivityMatchesUrls(activity, [url, finalUrl]);
+
+		return activity;
 	}
 
 	@bindThis

packages/backend/src/core/activitypub/misc/check-against-url.ts

@@ -0,0 +1,27 @@
+/*
+ * SPDX-FileCopyrightText: dakkar and sharkey-project
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+import type { IObject } from '../type.js';
+
+function getHrefFrom(one: IObject|string): string | undefined {
+	if (typeof(one) === 'string') return one;
+	return one.href;
+}
+
+export function assertActivityMatchesUrls(activity: IObject, urls: string[]) {
+	const idOk = activity.id !== undefined && urls.includes(activity.id);
+	if (idOk) return;
+
+	const url = activity.url;
+	if (url) {
+		// `activity.url` can be an `ApObject = IObject | string | (IObject
+		// | string)[]`, we have to look inside it
+		const activityUrls = Array.isArray(url) ? url.map(getHrefFrom) : [getHrefFrom(url)];
+		const goodUrl = activityUrls.find(u => u && urls.includes(u));
+
+		if (goodUrl) return;
+	}
+
+	throw new Error(`bad Activity: neither id(${activity?.id}) nor url(${JSON.stringify(activity?.url)}) match location(${urls})`);
+}