CVE-2026-54740: Lemmy: Lower-ranked federated moderator can remove higher-ranked moderators
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::ishighermodoradmincheck to enforce moderator rank, but CollectionRemove::verify in crates/apub/activities/src/community/collectionremove.rs only calls verifymodaction. 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 Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 0.19.19 - Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 1.0.0-alpha.18
Event History
Frequently Asked Questions
Which deployments are exposed to this issue?
Lemmy instances running versions before 0.19.19 or 1.0.0-alpha.18 are affected when communities have federated moderators. The vulnerable path is the handling of remote ActivityPub Remove activities.
What access does an attacker need?
An attacker needs to be a lower-ranked remote moderator in the affected community and able to send a signed ActivityPub Remove activity to the target instance. No user interaction is required.
What is the impact of successful exploitation?
A junior federated moderator can remove a higher-ranked moderator from the community moderator list. The issue affects integrity by bypassing the moderator-rank enforcement applied by the local API.
How can this be remediated?
Upgrade Lemmy to version 0.19.19 or 1.0.0-alpha.18. These versions add the missing rank check for the federated CollectionRemove handling path.
What can be done before upgrading?
The provided information identifies communities with federated moderators as the affected configuration. Until an upgrade is available, review moderator-list changes in such communities and limit reliance on lower-ranked remote moderator accounts where operationally possible.