Paper:
Backdoor attacks aim to manipulate a subset of training data by injecting adversarial triggers such that machine learning models trained on the tampered dataset will make arbitrarily (targeted) incorrect prediction on the testset with the same trigger embedded. While federated learning (FL) is capable of aggregating information provided by different parties for training a better model, its distributed learning methodology and inherently heterogeneous data distribution across parties may bring new vulnerabilities. In addition to recent centralized backdoor attacks on FL where each party embeds the same global trigger during training, we propose the distributed backdoor attack (DBA) -a novel threat assessment framework developed by fully exploiting the distributed nature of FL. DBA decomposes a global trigger pattern into separate local patterns and embed them into the training set of different adversarial parties respectively. Compared to standard centralized backdoors, we show that DBA is substantially more persistent and stealthy against FL on diverse datasets such as finance and image data. We conduct extensive experiments to show that the attack success rate of DBA is significantly higher than centralized backdoors under different settings. Moreover, we find that distributed attacks are indeed more insidious, as DBA can evade two state-of-the-art robust FL algorithms against centralized backdoors. We also provide explanations for the effectiveness of DBA via feature visual interpretation and feature importance ranking. To further explore the properties of DBA, we test the attack performance by varying different trigger factors, including local trigger variations (size, gap, and location), scaling factor in FL, data distribution, and poison ratio and interval. Our proposed DBA and thorough evaluation results shed lights on characterizing the robustness of FL.

Review:
This paper studies backdoor attacks under federated learning setting. To inject a certain backdoor pattern, existing work generate poisoning samples by blending the same pattern with different input samples. Even for federated learning where the adversary can control multiple parties, such as [1], all parties still use the same global backdoor pattern to generate poisoning samples locally. On the contrary, in this work, they decompose the global pattern into several small local patterns, and each adversarial party only uses a local pattern to generate poisoning samples. In their evaluation, they show that the backdoor attacks generated in this way are more effective, resilient to benign model parameter updates, and also survive better against existing defense algorithms against attacks in federated learning settings.
I think the topic studied in this paper is very important and meaningful, and I am convinced that by decomposing a global pattern into several smaller local pieces, the model parameter updates computed by each party should be more similar to benign updates and thus can better bypass the defense algorithms. Meanwhile, the evaluation is pretty comprehensive and it is good to see that the conducted backdoor attacks are effective. However, when there is no defense deployed in the training process, it is not intuitive to see why the proposed attack is more effective and persistent than the centralized attack, given that a smaller trigger usually results in a worse attack performance. Thus, I would like to see more possible explanation on it. Specifically, I have the following questions for clarification:
1. For the evaluation of DBA, I assume that there are 4 adversarial parties, controlling each of the 4 local triggers. When using centralized attacks, are there still 4 adversarial parties, although they share the same global trigger, or if there is only 1 adversarial party?
2. To evaluate A-S setting, I understand that it may be tricky to enable a fair comparison between the centralized attack and DBA. However, one explanation of why DBA is more persistent in this case is because the adversarial parameter updates happen 4x times compared to the centralized attack. Therefore, another baseline to check is to conduct centralized attacks with the same number of times as DBA, but each update includes 1/4 number of poisoning samples, so that the total number of poisoning samples included to compute the gradient update still stays the same.
3. Can the authors show if the decomposition is also useful for trigger patterns that are not necessarily regular shapes? For backdoor attacks, a line of work studies physical triggers, e.g., glasses in [2]. It is not natural to decompose such kind of patterns into several smaller pieces, unless the performance is significantly boosted.
4. Can the authors show concrete examples on how the attacks are generated? The details are especially unclear on LOAN. Specifically, which features are perturbed, what are the values assigned as the trigger, and what is the corresponding target label?
[1]  Bagdasaryan et al., How to backdoor federated learning.
[2] Chen et al., Targeted Backdoor Attacks on Deep Learning Systems Using Data Poisoning.
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Post-rebuttal comments
I appreciate the authors' great effort to address my concerns! I think the evaluation in the current version of the paper is pretty comprehensive and provides a valuable study, and I am happy to raise my score accordingly.
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