The increasing popularity of the federated learning (FL) framework due to its
success in a wide range of collaborative learning tasks also induces certain
security concerns. Among many vulnerabilities, the risk of Byzantine attacks is
of particular concern, which refers to the possibility of malicious clients
participating in the learning process. Hence, a crucial objective in FL is to
neutralize the potential impact of Byzantine attacks and to ensure that the
final model is trustable. It has been observed that the higher the variance
among the clients' models/updates, the more space there is for Byzantine
attacks to be hidden. As a consequence, by utilizing momentum, and thus,
reducing the variance, it is possible to weaken the strength of known Byzantine
attacks. The centered clipping (CC) framework has further shown that the
momentum term from the previous iteration, besides reducing the variance, can
be used as a reference point to neutralize Byzantine attacks better. In this
work, we first expose vulnerabilities of the CC framework, and introduce a
novel attack strategy that can circumvent the defences of CC and other robust
aggregators and reduce their test accuracy up to %33 on best-case scenarios in
image classification tasks. Then, we propose a new robust and fast defence
mechanism that is effective against the proposed and other existing Byzantine
attacks.