The linear dispersion relation for electromagnetic waves in a Vlasov plasma with unmagnetized ions and magnetized electrons undergoing an E¿B drift is derived and studied. Results show that electromagnetic effects reduce the growth rate of the modified two-stream instability while they spread the region of instability in wave vector space. The possible contribution of this instability to two bow shock parameters, the ratio of electric to magnetic field wave energy densities and the shock width, is discussed.