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