cert-revoke: fix permission check bypass (CVE-2016-5404)
The 'cert_revoke' command checks the 'revoke certificate'
permission, however, if an ACIError is raised, it then invokes the
'cert_show' command. The rational was to re-use a "host manages
certificate" check that is part of the 'cert_show' command, however,
it is sufficient that 'cert_show' executes successfully for
'cert_revoke' to recover from the ACIError continue. Therefore,
anyone with 'retrieve certificate' permission can revoke *any*
certificate and cause various kinds of DoS.
Fix the problem by extracting the "host manages certificate" check
to its own method and explicitly calling it from 'cert_revoke'.
Reviewed-By: Jan Cholasta <firstname.lastname@example.org>