  Federated learning (FL) attempts to train a global model by aggregating local
models from distributed devices under the coordination of a central server.
However, the existence of a large number of heterogeneous devices makes FL
vulnerable to various attacks, especially the stealthy backdoor attack.
Backdoor attack aims to trick a neural network to misclassify data to a target
label by injecting specific triggers while keeping correct predictions on
original training data. Existing works focus on client-side attacks which try
to poison the global model by modifying the local datasets. In this work, we
propose a new attack model for FL, namely Data-Agnostic Backdoor attack at the
Server (DABS), where the server directly modifies the global model to backdoor
an FL system. Extensive simulation results show that this attack scheme
achieves a higher attack success rate compared with baseline methods while
maintaining normal accuracy on the clean data.
