  Recently, researchers have successfully employed Graph Neural Networks (GNNs)
to build enhanced recommender systems due to their capability to learn patterns
from the interaction between involved entities. In addition, previous studies
have investigated federated learning as the main solution to enable a native
privacy-preserving mechanism for the construction of global GNN models without
collecting sensitive data into a single computation unit. Still, privacy issues
may arise as the analysis of local model updates produced by the federated
clients can return information related to sensitive local data. For this
reason, experts proposed solutions that combine federated learning with
Differential Privacy strategies and community-driven approaches, which involve
combining data from neighbor clients to make the individual local updates less
dependent on local sensitive data. In this paper, we identify a crucial
security flaw in such a configuration, and we design an attack capable of
deceiving state-of-the-art defenses for federated learning. The proposed attack
includes two operating modes, the first one focusing on convergence inhibition
(Adversarial Mode), and the second one aiming at building a deceptive rating
injection on the global federated model (Backdoor Mode). The experimental
results show the effectiveness of our attack in both its modes, returning on
average 60% performance detriment in all the tests on Adversarial Mode and
fully effective backdoors in 93% of cases for the tests performed on Backdoor
Mode.
