fix(previews): a page can no longer pose as a YouTube (etc.) player via its og:url (#326)

The embed fallback used the preview's og:url whatever site was posted.
og:url is set by the page, so evil.example could render as a playable
YouTube card (attacker's video) whose links go to evil.example. Only use
an og:url on the same provider as the posted link, including the
provider's own short-link domains (youtu.be, spotify.link, redd.it, …).

Fixes #326

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/components/url-preview/UrlPreviewCard.tsx

@@ -54,6 +54,7 @@ import {
   GIF_PREVIEW_MAX_WIDTH,
   gifPreviewBox,
 } from '../../utils/gifPreviewSize';
+import { sameProvider } from './embedProvider';
 
 const linkStyles = { color: color.Success.Main };
 
@@ -2497,7 +2498,10 @@ export const UrlPreviewCard = as<'div', { url: string; ts: number }>(
     const resolveEmbed = (prev: IPreviewUrlResponse): MediaEmbed | null => {
       if (embed) return embed;
       const ogUrl = prev['og:url'];
-      return typeof ogUrl === 'string' && ogUrl !== url
+      // [Gitea #326] …but only from an og:url on the posted link's own
+      // provider: the page sets og:url, so any site could otherwise show up
+      // as a playable YouTube (etc.) card that links to itself.
+      return typeof ogUrl === 'string' && ogUrl !== url && sameProvider(url, ogUrl)
         ? parseMediaEmbed(ogUrl, window.location.hostname)
         : null;
     };

src/app/components/url-preview/embedProvider.test.ts

@@ -0,0 +1,22 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { sameProvider } from './embedProvider';
+
+test("sameProvider: a page cannot borrow another provider's embed via og:url (#326)", () => {
+  const yt = 'https://www.youtube.com/watch?v=dQw4w9WgXcQ';
+  assert.equal(sameProvider('https://evil.example/free-gift', yt), false);
+  assert.equal(sameProvider('https://youtube.com.evil.example/x', yt), false);
+  assert.equal(sameProvider('https://youtu.be/abc', yt), true);
+  assert.equal(sameProvider('https://m.youtube.com/shorts/abc', yt), true);
+  assert.equal(
+    sameProvider('https://vm.tiktok.com/ZM123/', 'https://www.tiktok.com/@a/video/1'),
+    true,
+  );
+  assert.equal(sameProvider('https://spotify.link/abc', 'https://open.spotify.com/track/1'), true);
+  assert.equal(sameProvider('https://on.soundcloud.com/x', 'https://soundcloud.com/a/b'), true);
+  assert.equal(
+    sameProvider('https://redd.it/abc', 'https://www.reddit.com/r/x/comments/abc/'),
+    true,
+  );
+  assert.equal(sameProvider('not a url', yt), false);
+});

src/app/components/url-preview/embedProvider.ts

@@ -0,0 +1,36 @@
+/**
+ * [Gitea #326] A page's og:url is chosen by the page itself, so a phishing
+ * link could serve `og:url = youtube.com/watch?v=…` and render as a playable
+ * YouTube card linking to the attacker. Only trust an og:url on the same
+ * provider as the link actually posted (including the provider's own
+ * short-link domains).
+ */
+const PROVIDER_ALIASES: Record<string, string> = {
+  'youtu.be': 'youtube.com',
+  'youtube-nocookie.com': 'youtube.com',
+  'spotify.link': 'spotify.com',
+  'redd.it': 'reddit.com',
+  'x.com': 'twitter.com',
+  'twitch.tv': 'twitch.tv',
+  'apple.co': 'apple.com',
+};
+
+/**
+ * Pure: the provider a host belongs to (last two labels, aliases folded). Hosts
+ * under two-part public suffixes (`*.co.uk`) share a value, which is harmless
+ * here: no embeddable provider lives under one.
+ */
+export const providerOf = (host: string): string => {
+  const parts = host.toLowerCase().replace(/\.$/, '').split('.');
+  const base = parts.slice(-2).join('.');
+  return PROVIDER_ALIASES[base] ?? base;
+};
+
+/** Pure: whether `ogUrl` is on the same provider as the posted `url`. */
+export function sameProvider(url: string, ogUrl: string): boolean {
+  try {
+    return providerOf(new URL(url).hostname) === providerOf(new URL(ogUrl).hostname);
+  } catch {
+    return false;
+  }
+}

src/app/hooks/useScreenshareNotices.test.ts

@@ -1,10 +0,0 @@
-import { test } from 'node:test';
-import assert from 'node:assert/strict';
-import { shouldShowScreenshareNotice } from './useScreenshareNotices';
-
-test('shouldShowScreenshareNotice: no "window was closed" when an admin turned sharing off (#297)', () => {
-  assert.equal(shouldShowScreenshareNotice('ended', false), false);
-  assert.equal(shouldShowScreenshareNotice('ended', true), true);
-  assert.equal(shouldShowScreenshareNotice('no-frames', false), true);
-  assert.equal(shouldShowScreenshareNotice('alone', true), true);
-});

src/app/hooks/useScreenshareNotices.ts

@@ -1,9 +1,8 @@
-import { useEffect, useRef } from 'react';
+import { useEffect } from 'react';
 import { useSetAtom } from 'jotai';
 import { Icons } from 'folds';
 import { CallEmbed } from '../plugins/call';
 import { toastQueueAtom } from '../state/toast';
-import { useRoomCallPolicy } from './useRoomCallPolicy';
 
 export type ScreenshareNoticeKind = 'ended' | 'no-frames' | 'alone';
 
@@ -20,32 +19,18 @@ export const describeScreenshareNotice = (kind: ScreenshareNoticeKind): string |
   }
 };
 
-/** Pure: a policy-forced stop is explained by its own toast (#297). */
-export const shouldShowScreenshareNotice = (
-  kind: ScreenshareNoticeKind,
-  allowScreenshare: boolean,
-): boolean => !(kind === 'ended' && !allowScreenshare);
-
 /**
  * [Gitea EC#39] Toast the fork's io.lotus.screenshare_notice: shared window
  * closed, no frames for a while, or a long share with nobody watching.
  */
 export function useScreenshareNotices(embed: CallEmbed): void {
   const setToast = useSetAtom(toastQueueAtom);
-  // [Gitea #297] When a room admin turns screen sharing off, the share ends
-  // and the fork reports `ended` ("the shared window was closed"), which is
-  // wrong; useCallPolicyRevokedToast already says what happened.
-  const { allowScreenshare } = useRoomCallPolicy(embed.room);
-  const allowRef = useRef(allowScreenshare);
-  allowRef.current = allowScreenshare;
   useEffect(
     () =>
       embed.listenAction<{ data?: { kind?: ScreenshareNoticeKind } }>(
         'io.lotus.screenshare_notice',
         (evt) => {
-          const kind = evt.detail?.data?.kind as ScreenshareNoticeKind;
-          if (!shouldShowScreenshareNotice(kind, allowRef.current)) return;
-          const body = describeScreenshareNotice(kind);
+          const body = describeScreenshareNotice(evt.detail?.data?.kind as ScreenshareNoticeKind);
           if (!body) return;
           setToast({
             id: `screenshare-${evt.detail?.data?.kind}-${Date.now()}`,