Authenticating a user means verifying his identity. Today, authentication systems are used for all kind of applications, in various forms: from traditional secrecy-based methods to more modern biometrics-based ones. Behavioral authentication in particular, has become very relevant as a mean of continuously verifying a user's genuineness. These systems are built on top of machine learning algorithms, and as such, they are subject to adversary attacks. Evasion attacks rely on generating adversarial instances capable of evading a classifier with small modifications. While adversarial instances have been successfully generated for differentiable models, this is not true for tree ensembles, for which the literature is very limited.