In the nitration of benzene with a mixture of concentrated $\mathrm{HNO}{}_3$ and concentrated $\mathrm{H}{}_2\mathrm{SO}{}_4$, the attacking electrophile is

Sulfuric acid is the stronger acid and protonates $\mathrm{HNO}{}_3$, which loses water to give the nitronium ion: $\mathrm{HNO}{}_3+2\mathrm{H}{}_2\mathrm{SO}{}_4\to \mathrm{NO}{}_2{}^{+}+\mathrm{H}{}_3\mathrm{O}{}^{+}+2\mathrm{HSO}{}_4{}^{-}$. The nitronium ion is the electrophile that attacks the benzene ring.