In recent years, information hiding method has become a research hotspot in information security field. The user authentication is the vital processing in network security. This paper proposes a data hiding method using the Discrete Cosine Transform (DCT) algorithm to store network authentication information into RGB images. The authentication datasets are converted into the frequency domain and embedded into the raw picture, which is almost unobservable to human eyes. After the encrypted image is delivered to the destination node, the authentication is realized by decoding the hidden DCT coefficients in the images. Due to the invisible authentication information, the proposed DCT-based authentication method is more secure than the traditional bar code and Quick Response (QR) code.