An Embarrassingly Simple Defense Against Backdoor Attacks On SSL

Self Supervised Learning (SSL) has emerged as a powerful paradigm to tackle data landscapes with absence of human supervision. The ability to learn meaningful tasks without the use of labeled data makes SSL a popular method to manage large chunks of data in the absence of labels. However, recent work indicates SSL to be vulnerable to backdoor attacks, wherein models can be controlled, possibly maliciously, to suit an adversary's motives. Li et. al (2022) introduce a novel frequency-based backdoor attack: CTRL. They show that CTRL can be used to efficiently and stealthily gain control over a victim's model trained using SSL. In this work, we devise two defense strategies against frequency-based attacks in SSL: One applicable before model training and the second to be applied during model inference. Our first contribution utilizes the invariance property of the downstream task to defend against backdoor attacks in a generalizable fashion. We observe the ASR (Attack Success Rate) to reduce by over 60% across experiments. Our Inference-time defense relies on evasiveness of the attack and uses the luminance channel to defend against attacks. Using object classification as the downstream task for SSL, we demonstrate successful defense strategies that do not require re-training of the model. Code is available at https://github.com/Aryan-Satpathy/Backdoor.