CVE-2026-54740
MediumCVSS 6.5Summary
Lemmy before versions 0.19.19 and 1.0.0-alpha.18 allows a lower-ranked moderator to remove a higher-ranked moderator by sending a signed ActivityPub Remove activity. The verification does not check moderator rank, enabling a junior moderator to strip senior moderators from the community moderator list.
Risk Assessment
An attacker can unauthorizedly remove moderators, potentially leading to community takeover and compromise of management integrity.
Recommendation
Upgrade Lemmy to version 0.19.19 or 1.0.0-alpha.18, which include the fix.
Original NVD description (English source)
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.