  Aggregate signatures allow anyone to combine different signatures signed by
different signers on different messages into a single short signature. An ideal
aggregate signature scheme is an identity-based aggregate signature (IBAS)
scheme that supports full aggregation since it can reduce the total transmitted
data by using an identity string as a public key and anyone can freely
aggregate different signatures. Constructing a secure IBAS scheme that supports
full aggregation in bilinear maps is an important open problem. Recently, Yuan
{\it et al.} proposed an IBAS scheme with full aggregation in bilinear maps and
claimed its security in the random oracle model under the computational
Diffie-Hellman assumption. In this paper, we show that there exists an
efficient forgery attacker on their IBAS scheme and their security proof has a
serious flaw.
