  Federated learning (FL) is an emerging machine learning paradigm, in which
clients jointly learn a model with the help of a cloud server. A fundamental
challenge of FL is that the clients are often heterogeneous, e.g., they have
different computing powers, and thus the clients may send model updates to the
server with substantially different delays. Asynchronous FL aims to address
this challenge by enabling the server to update the model once any client's
model update reaches it without waiting for other clients' model updates.
However, like synchronous FL, asynchronous FL is also vulnerable to poisoning
attacks, in which malicious clients manipulate the model via poisoning their
local data and/or model updates sent to the server. Byzantine-robust FL aims to
defend against poisoning attacks. In particular, Byzantine-robust FL can learn
an accurate model even if some clients are malicious and have Byzantine
behaviors. However, most existing studies on Byzantine-robust FL focused on
synchronous FL, leaving asynchronous FL largely unexplored. In this work, we
bridge this gap by proposing AFLGuard, a Byzantine-robust asynchronous FL
method. We show that, both theoretically and empirically, AFLGuard is robust
against various existing and adaptive poisoning attacks (both untargeted and
targeted). Moreover, AFLGuard outperforms existing Byzantine-robust
asynchronous FL methods.
