Title: Are vision transformers more robust than CNNs for Backdoor attacks?. Abstract: Transformer architectures are based on a self-attention mechanism that processes images as a sequence of patches. As their design is quite different compared to CNNs, it is interesting to study if transformers are vulnerable to backdoor attacks and how different transformer architectures affect attack success rates. Backdoor attacks happen when an attacker poisons a small part of the training images with a specific trigger or backdoor which will be activated later. The model performance is good on clean test images, but the attacker can manipulate the decision of the model by showing the trigger on an image at test time. In this paper, we perform a comparative study of state-of-the-art architectures through the lens of backdoor robustness, specifically how attention mechanisms affect robustness. We show that the popular vision transformer architecture (ViT) is the least robust architecture and ResMLP, which belongs to a class called Feed Forward Networks (FFN), is the most robust one to backdoor attacks among state-of-the-art architectures. We also find an intriguing difference between transformers and CNNs – interpretation algorithms effectively highlight the trigger on test images for transformers but not for CNNs. Based on this observation, we find that a test-time image blocking defense reduces the attack success rate by a large margin for transformers. We also show that such blocking mechanisms can be incorporated during the training process to improve robustness even further. We believe our experimental findings will encourage the community to understand the building block components in  developing novel architectures robust to backdoor attacks.