Supernova remnants are known as efficient particle accelerators due strong non-thermal radiation emitted by self-produced relativistic electrons, protons and ions. Diffusive shock acceleration (DSA) operating in these shocks is believed to be responsible for acceleration of high energy particles. In order for DSA to work, a particle should have a gyroradus larger than the finite width of a shock. Therefore, thermal electrons require a substantial pre-acceleration before they can cross the shock within one gyration and be accelerated via DSA. Here we use particle-in-cell simulations to study microphysics of electron acceleration in oblique high Mach number shocks. In this case, fast electrons can escape to the shock upstream, modifying the shock foot to a region called the electron foreshock. We find that the observed electron-beam instabilities agree very well with the predictions of a linear dispersion analysis: the electrostatic electron-acoustic instability dominates in the outer region of the foreshock, while the denser electron beams in the inner foreshock drive the gyroresonant oblique-whistler instability. Both foreshock instabilities play a crucial role in production of nonthermal electrons via resonant interaction and stochastic scattering. As a result, the downstream spectrum of electrons is characterized with a pronounced nonthermal tail, making the injection of electrons into DSA possible, thus enabling electron acceleration to higher energies.