Logit-Margin Repulsion for Backdoor Defense

Backdoor attacks pose a significant threat to deep neural networks. Recent studies have shown that model compression, such as quantization and pruning, can be exploited by attackers to implant conditional backdoors. Such backdoors remain dormant in the original model but are activated after the model undergoes specific operations, making them highly stealthy and difficult to detect. Traditional defense methods struggle to counter this type of attack, while defenses specifically designed for conditional backdoors also have difficulty handling traditional backdoor attacks. To address these challenges, we propose a universal defense method, termed Logit Margin Repulsion (LMR). LMR uses a small set of clean samples and combines selective crossentropy with a logit-margin constraint to enlarge the gap between the backdoor class and benign classes. It then removes channels associated with backdoor behavior through selective pruning, thereby achieving strong backdoor purification. Extensive experiments on a variety of CNNs and Vision Transformers demonstrate that, even with an extremely limited amount of clean data (0.1%), LMR can effectively mitigate both traditional and conditional backdoor attacks. The implementation is publicly available on https://github.com/Trusted-LLM/LMR.