Paper:
Neural network (NN) trojaning attack is an emerging and important attack that can broadly damage the system deployed with NN models. Different from adversarial attacks, it hides malicious functionality in the weight parameters of NN models. Existing studies have explored NN trojaning attacks in some small datasets for specific domains, with limited numbers of fixed target classes. In this paper, we propose a more powerful trojaning attack method for large models, which outperforms existing studies in capability, generality, and stealthiness. First, the attack is programmable that the malicious misclassification target is not fixed and can be generated on demand even after the victim's deployment. Second, our trojaning attack is not limited in a small domain; one trojaned model on a large-scale dataset can affect applications of different domains that reuses its general features. Third, our trojan shows no biased behavior for different target classes, which makes it more difficult to defend.

Review:
This paper proposed a "learnable" trojan by training a neural network that takes a sample (e.g. an image) as an input and generates a programmable trigger pattern as an output. The authors argue that the proposed method can support dynamic and out scope target classes, which are particularly applicable to backdoor attacks in the transfer learning setting. The authors conducted experiments on large-scale models (ImageNet) in two settings: (1) outsourced training attack;  (2) transfer learning attack.
Although the idea of making a backdoor attack more robust (e.g. more transferrable) and programmable is interesting, I don't think the current results fully substantiate the claimed benefits. Below are my concerns:
1. Lack of performance comparison: in the outsourced training attack, the attack success rate is quite low ( the best top-1 attack rate is ~50% on VGG). In the existing literature, the backdoor attack success rate can be made nearly 100%. This makes me wonder whether the low attack success rate only occurs in the proposed attacking method. Since there are no results from existing attacks, there is no way to evaluate how good the proposed attack is.
2. How about small dataset/model? I understand that the authors want to emphasize the scalability of their attack to large models like ImageNet. However, there are no comparisons on ImageNet (my comment 1). The authors are suggested to compare performance on standard datasets in backdoor attack literature (e.g. CIFAR-10, traffic sign)
3. It was unclear how "dynamic" the proposed method can be. Based on the attack formulation in equation (3), in order to make the attack "dynamic" in terms of changing different target classes, the attackers need to train a neural trojan network for every target class, which does not seem to be dynamic to me. Can the authors further justify the advantage of the dynamic feature in the proposed attack? And I have concerns about how many target classes an attacker can "dynamically" change. Some experiments showing the number of target classes vs attack performance and clean data accuracy will be very helpful.
4. The defense argument against detection methods is weak. Unless the authors can show the proposed attack has the ability to simultaneously backdoor all possible target classes, simply arguing the attack is dynamic and thus can evade detection is not convincing, not to mention in the backdoor setting, attacker should make the first move before the defender takes action.
*** Post-rebuttal comments
I thank the authors for the clarification. However, I feel my comments have not been fully addressed, especially on the part on justifying 50% attack success rate on VGG should be considered as significant in the considered setup. Without any valid comparisons, I find it difficult to assess the contributions. In addition, the authors did not add new empirical evidence regarding my questions but mainly re-iterated the applicability of the proposed method, so I will remain my review rating.
***