  Recently, NLP has seen a surge in the usage of large pre-trained models.
Users download weights of models pre-trained on large datasets, then fine-tune
the weights on a task of their choice. This raises the question of whether
downloading untrusted pre-trained weights can pose a security threat. In this
paper, we show that it is possible to construct ``weight poisoning'' attacks
where pre-trained weights are injected with vulnerabilities that expose
``backdoors'' after fine-tuning, enabling the attacker to manipulate the model
prediction simply by injecting an arbitrary keyword. We show that by applying a
regularization method, which we call RIPPLe, and an initialization procedure,
which we call Embedding Surgery, such attacks are possible even with limited
knowledge of the dataset and fine-tuning procedure. Our experiments on
sentiment classification, toxicity detection, and spam detection show that this
attack is widely applicable and poses a serious threat. Finally, we outline
practical defenses against such attacks. Code to reproduce our experiments is
available at https://github.com/neulab/RIPPLe.
