Title: Rethinking Backdoor Data Poisoning Attacks in the Context of Semi-Supervised Learning. Abstract: Semi-supervised learning methods can train high-accuracy machine learning models with a fraction of the labeled training samples required for traditional supervised learning. Such methods do not typically involve close review of the unlabeled training samples, making them tempting targets for data poisoning attacks. In this paper we investigate the vulnerabilities of semi-supervised learning methods to backdoor data poisoning attacks on the unlabeled samples. We show that a simple poisoning attack using adversarially perturbed samples is highly effective - achieving an average attack success rate of 93.6%. We introduce a generalized attack framework targeting semi-supervised learning methods to better understand and exploit their limitations and to motivate future defense strategies.