Missing authorization in the REST role-management routes of Progressive Robot hMailServer 6.3.4 through 6.3.6 allows a remote authenticated user whose credential is confined to named domains to modify a custom role that accounts in other domains hold, and to create custom roles and delete any that no account holds. The routes that create, update and delete a custom role require the manage_roles permission, which only the serveradmin role holds; but a full-scope API key issued for named domains, and serveradmin assigned with a domain list, are serveradmin confined to those domains and hold manage_roles. The server's authorization gate did not refuse the three role routes to a confined credential and the route named no domain, so a credential issued to one tenant could raise or lower the permissions of a custom role assigned across other tenants - granting another domain's role every permission over its mailboxes and settings, or removing what it needs. A custom role may not hold administer_server, administer_domain or manage_roles, so the confined credential cannot make itself or another a server administrator by creating or rewriting a custom role.
Upgrade to hMailServer 6.3.7, whose authorization gate refuses the create, update and delete role routes to a credential confined to named domains, while leaving the read of roles unchanged. Until then, on a multi-tenant server, do not issue a full-scope API key - or assign serveradmin - confined to named domains to a party not trusted with the whole server; keep the REST API off (RestApiPort=0, the default) where it is not needed; and review custom roles' permissions for changes a confined credential should not have made (after upgrading as well, since an upgrade does not undo a change already made).