A fixed-length CBC-MAC uses zero block as IV (initialization vector) to prevent from changing the first block.
i.e. tag $t$ = $F(m_x \oplus ... F(m_3 \oplus F(m_2 \oplus F(m_1)))...)$
My question is, if the first block is used as IV, then is it secure (unforgeable), and why? (not for encryption/confidentiality, and just for authentication)
i.e. tag $t$ = $F(m_x \oplus ... F(m_4 \oplus F(m_3 \oplus F(m_1 \oplus m_2)))...)$