Plane is an open-source project management tool. Prior to 1.4.0, self-hosted Plane deployments allow unauthenticated GET requests to api/workspaces/{slug}/invitations/{pk}/join/ through WorkspaceJoinEndpoint, whose WorkSpaceMemberInviteSerializer exposes the secret invitation token directly and through get_invite_link. An external attacker can retrieve a token intended for another user and submit it to the invitation acceptance flow, joining the workspace without receiving the invitation and gaining the role assigned to the invite. This issue is fixed in version 1.4.0.