Incorrect authorization in the REST administration API of Progressive Robot hMailServer 6.3.4 through 6.3.6 allows a remote authenticated user whose credential holds the manage_roles permission in named domains - a full-scope API key issued for those domains, or an account assigned the serveradmin role with a list of domains - to obtain the server administrator's authority. An account's admin level gives it authority, and a role assigned to it replaces that authority, which is how an administrator narrows an account whose admin level is the server administrator's. The route that takes a role away (DELETE /api/v1/role-assignments/{address}) checked only that the credential takes in what the account held at the time, not what it would hold afterwards, so such a credential could take a narrowing role away from an account in its domains whose admin level is the server administrator's, returning it to the server administrator's authority, and, having set that account's password while it held the narrower role, sign in as the server administrator.
Upgrade to hMailServer 6.3.7, in which a role is given or taken away, over the REST and the COM APIs, only by a credential that holds manage_roles in the account's domain and takes in both what the account holds now and what it will hold afterwards - with no role left, the authority of its admin level - so a credential confined to named domains is refused taking a role away from an account whose admin level is the server administrator's. Until then, give an account that a role narrows the admin level its role intends rather than the server administrator's, do not issue a full-scope API key, or assign serveradmin, confined to named domains to a party not trusted with the whole server, and review the role assignments and the accounts whose admin level is the server administrator's (after upgrading as well, since an upgrade does not give back a role already taken away).