GHSA-jmxc-hhwx-gvv3

## Summary `read_multi_community()` does not enforce the private-instance setting. On a private instance, an unauthenticated visitor can read multi-community names, titles, summaries, sidebars, owner identities, and member community lists. ## Details Other read handlers load `local_site` and call `check_private_instance()` before returning data to unauthenticated callers. `read_multi_community()` does not call that helper: ```rust pub async fn read_multi_community( Query(data): Query<GetMultiCommunity>, context: Data<LemmyContext>, local_user_view: Option<LocalUserView>, ) -> LemmyResult<Json<GetMultiCommunityResponse>> { let my_person_id = local_user_view.as_ref().map(|l| l.person.id); let id = resolve_multi_community_identifier(&data.name, data.id, &context, &local_user_view) .await? .ok_or(LemmyErrorType::NoIdGiven)?; let multi_community_view = MultiCommunityView::read(&mut context.pool(), id, my_person_id).await?; ``` `get_community()`, `list_posts()`, `list_comments()`, `read_person()`, `search()`, and `resolve_object()` all enforce the private-instance guard. ## Proof of Concept The script creates a multi-community whose metadata contains a marker, turns on `private_instance`, confirms a guarded control endpoint blocks unauthenticated callers, then reads the same multi-community over `GET /multi_community` without authentication. ```python #!/usr/bin/env python3 import json, random, string import requests BASE = "http://127.0.0.1:8536/api/v4" ADMIN_USER = "lemmy" ADMIN_PASS = "lemmylemmy" def api(method, path, token=None, **kw): h = kw.pop("headers", {}) if token: h["Authorization"] = "Bearer " + token return requests.request(method, BASE + path, headers=h, **kw) suffix = "multi" + "".join(random.choice(string.ascii_lowercase) for _ in range(6)) secret = "SECRET_MULTI_" + suffix admin = api("POST", "/account/auth/login", json={"username_or_email": ADMIN_USER, "password": ADMIN_PASS}).json()["jwt"] # Create