  In AFRICACRYPT 2010, Abdalla et al. first proposed a slight modification to
the computations steps of the BD protocol, called mBD+P. Then they extended
mBD+P protocol into mBD+S protocol. In this paper, we show that both of mBD+P
and mBD+S protocols are vulnerable to malicious insiders attack. Further, we
propose a simple countermeasure against this attack.
