Lemmy: Lower-ranked federated moderator can remove higher-ranked moderators

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-54740
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-19 21:16:58 UTC
Updated2026-08-20 20:17:35 UTC
DescriptionLemmy 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.

Risk And Classification

Primary CVSS: v3.1 6.5 MEDIUM from [email protected]

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N

EPSS: 0.003050000 probability, percentile 0.232600000 (date 2026-08-23)

Problem Types: CWE-862 | CWE-862 CWE-862: Missing Authorization

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary6.5MEDIUMCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N
3.1CNADECLARED6.5MEDIUMCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA LemmyNet Lemmy affected < 0.19.19 Not specified
CNA LemmyNet Lemmy affected >= 1.0.0-alpha.0, < 1.0.0-alpha.18 Not specified

References

ReferenceSourceLinkTags
join-lemmy.org/news/2026-06-09_-_Lemmy_Release_v0.19.19 [email protected] join-lemmy.org
github.com/LemmyNet/lemmy/pull/6475 [email protected] github.com
github.com/LemmyNet/lemmy/pull/6478 [email protected] github.com
github.com/LemmyNet/lemmy/releases/tag/1.0.0-alpha.18 [email protected] github.com
github.com/LemmyNet/lemmy/security/advisories/GHSA-xmpx-2j2f-c7g7 134c704f-9b21-4f2e-91b3-4a467353bcc0 github.com
github.com/LemmyNet/lemmy/releases/tag/0.19.19 [email protected] github.com
github.com/LemmyNet/lemmy/commit/ede1d348344ca2027fd1bc6a0cab25575c0c3a4c [email protected] github.com
github.com/LemmyNet/lemmy/commit/d127e714a66f34f6b9c8ea15f20097ac907cdf28 [email protected] github.com
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