In 2008, Cao et al. proposed an identity-based multi-user broadcast authentication scheme (called IMBAS) for wireless sensor networks. The IMBAS consists of one elliptic curve-based signature scheme, called variant of Bellare-Namprempre-Neven' Identity-Based Signature (vBNN-IBS), for user broadcast authentication; a new Schnorr signature with partial message recovery for sink broadcast authentication; and a password-based private key protection to eliminate the threat caused by possible user device compromise. Compared with its counterparts, IMBAS owns better performance in terms of security, scalability, and efficiency. Unfortunately, this letter will show that (1) the password-based protection scheme and the vBNN-IBS scheme are not secure—a compromised user device will disclose the user private key—and (2) the new Schnorr signature with partial message recovery is vulnerable to existential forgery attack. Copyright © 2012 John Wiley & Sons, Ltd.