  Federated learning enables multiple users to build a joint model by sharing
their model updates (gradients), while their raw data remains local on their
devices. In contrast to the common belief that this provides privacy benefits,
we here add to the very recent results on privacy risks when sharing gradients.
Specifically, we investigate Label Leakage from Gradients (LLG), a novel attack
to extract the labels of the users' training data from their shared gradients.
The attack exploits the direction and magnitude of gradients to determine the
presence or absence of any label. LLG is simple yet effective, capable of
leaking potential sensitive information represented by labels, and scales well
to arbitrary batch sizes and multiple classes. We mathematically and
empirically demonstrate the validity of the attack under different settings.
Moreover, empirical results show that LLG successfully extracts labels with
high accuracy at the early stages of model training. We also discuss different
defense mechanisms against such leakage. Our findings suggest that gradient
compression is a practical technique to mitigate the attack.
