Although high-energy cosmic rays (HECRs) and gamma-ray bursts (GRBs) are the mostenergetic phenomena occurring in the universe, their origin are important enigmas in thefield of astrophysics. Today, the most studied scenario that attempts to explain them isknown as the Fireball Model. This theory assumes that the particles are accelerated by ashock developing in the interior of a relativistic plasma from a supernova (SN). Thefilamentation (sometimes called “Weibel”) instability is believed to mediate collisionlessshock formation from the collision of two plasma shells. It has been known for long that aflow aligned magnetic field can completely cancel this instability. In this work, weanalyze the robustness of the filamentation instability which develops inside a plasmaimmersed in an arbitrarily oriented magnetic field.