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We perform a complete study of the gravitational lensing effect beyond theBorn approximation on the Cosmic Microwave Background (CMB) anisotropies usinga multiple-lens raytracing technique through cosmological N-body simulations ofthe DEMNUni suite. The impact of second order effects accounting for thenon-linear evolution of large-scale structures is evaluated propagating for thefirst time the full CMB lensing jacobian together with the light raystrajectory. We carefully investigate the robustness of our approach againstseveral numerical effects in the raytracing procedure and in the N-bodysimulation itself, and find no evidence of large contaminations. We discuss theimpact of beyond-Born corrections on lensed CMB observables, and compare ourresults with recent analytical predictions that appeared in the literature,finding a good agreement, and extend these results to smaller angular scales.We measure the gravitationally-induced CMB polarization rotation that appearsat second order, and compare this result with the latest analyticalpredictions. We then present the detection prospect of beyond-Born effects withthe future CMB-S4 experiment. We show that corrections to the temperature powerspectrum can be measured only if a good control of the extragalacticforegrounds is achieved. Conversely, the beyond-Born corrections on E andB-modes power spectra will be much more difficult to detect.

CMB weak-lensing beyond the Born approximation: a numerical approach