CVE-2026-54740 Lemmy: Lower-ranked federated moderator can remove higher-ranked moderators
A lower-ranked remote moderator can remove higher-ranked moderators in Lemmy communities due to missing rank checks in ActivityPub handling.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-54740?
CVE-2026-54740 is a medium-severity missing authorization vulnerability in LemmyNet lemmy, published on 19 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
Lemmy is a link aggregator and forum for the fediverse. Prior to 0.19.19 and 1.0.0-alpha.18, a lower-ranked remote moderator can remove a higher-ranked moderator by sending a signed ActivityPub Remove activity to the target instance. The local API uses LocalUser::is_higher_mod_or_admin_check to enforce moderator rank, but CollectionRemove::verify in crates/apub/activities/src/community/collection_remove.rs only calls verify_mod_action. CollectionRemove::receive dereferences self.object as an ApubPerson, creates a CommunityModeratorForm, and calls CommunityActions::leave without checking that the actor outranks the moderator identified by the object field. In communities with federated moderators, a junior moderator can therefore strip senior moderators from the community moderator list even though the local API rejects the same action. This issue is fixed in versions 0.19.19 and 1.0.0-alpha.18.
Weakness: CWE-862
Which versions are affected by CVE-2026-54740?
How do I fix CVE-2026-54740?
No fix has been published for CVE-2026-54740 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 CVE-2026-54740 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-54740 exists.
Sources
Also known as GHSA-xmpx-2j2f-c7g7.