AbstractDeniable authentication, which was introduced to realize secure online negotiation, has been an extensive topic of research. Wang et al. proposed an efficient deniable authentication protocol based on ElGamal cryptography. However, in this article, we show that Wang et al.'s protocol (WLT protocol) is vulnerable to the person‐in‐the‐middle (PIM) attack. An adversary who launches a PIM attack can impersonate a valid user to communicate with the verifier, which will eventually lead to the failure of an online negotiation. To mitigate this security breach, we propose an improved deniable authentication protocol based on Wang et al.'s scheme. Further, a detailed security analysis is conducted to verify the proposed scheme. © 2006 Wiley Periodicals, Inc. NETWORKS, Vol. 48(4), 179–181 2006