On Feasibility of Server-side Backdoor Attacks on Split Learning

Split learning is a collaborative learning design that allows several participants (clients) to train a shared model while keeping their datasets private. In split learning, the network is split into two halves: clients have the initial part until the cut layer, and the remaining part of the network is on the server side. In the training process, clients feed the data into the first part of the network and send the output (smashed data) to the server, which uses it as the input for the remaining part of the network. Recent studies demonstrate that collaborative learning models, specifically federated learning, are vulnerable to security and privacy attacks such as model inference and backdoor attacks. While there have been studies regarding inference attacks on split learning, it has not yet been tested for backdoor attacks. This paper performs a novel backdoor attack on split learning and studies its effectiveness. Despite traditional backdoor attacks done on the client side, we inject the backdoor trigger from the server side. We provide two attack methods: one using a surrogate client and another using an autoencoder to poison the model via incoming smashed data and its outgoing gradient toward the innocent participants. The results show that despite using strong patterns and injection methods, split learning is highly robust and resistant to such poisoning attacks. While we get the attack success rate of 100% as our best result for the MNIST dataset, in most of the other cases, our attack shows little success when increasing the cut layer.