CVE-2026-54740: Lemmynet Lemmy

Medium severity, CVSS 6.5. EPSS: 0.4% chance of exploitation in the next 30 days.

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.

Affected products

• Lemmynet Lemmy: before 0.19.19 (fixed in 0.19.19); from 1.0.0-alpha.0, before 1.0.0-alpha.18 (fixed in 1.0.0-alpha.18)

Published 2026-08-19. Last modified 2026-09-09.