  Continual learning algorithms are typically exposed to untrusted sources that
contain training data inserted by adversaries and bad actors. An adversary can
insert a small number of poisoned samples, such as mislabeled samples from
previously learned tasks, or intentional adversarial perturbed samples, into
the training datasets, which can drastically reduce the model's performance. In
this work, we demonstrate that continual learning systems can be manipulated by
malicious misinformation and present a new category of data poisoning attacks
specific for continual learners, which we refer to as {\em Poisoning Attacks
Against Continual Learners} (PACOL). The effectiveness of labeling flipping
attacks inspires PACOL; however, PACOL produces attack samples that do not
change the sample's label and produce an attack that causes catastrophic
forgetting. A comprehensive set of experiments shows the vulnerability of
commonly used generative replay and regularization-based continual learning
approaches against attack methods. We evaluate the ability of label-flipping
and a new adversarial poison attack, namely PACOL proposed in this work, to
force the continual learning system to forget the knowledge of a learned
task(s). More specifically, we compared the performance degradation of
continual learning systems trained on benchmark data streams with and without
poisoning attacks. Moreover, we discuss the stealthiness of the attacks in
which we test the success rate of data sanitization defense and other outlier
detection-based defenses for filtering out adversarial samples.
