FeatureTrojan: Boosting stealthy and steady backdoor attacks with feature poisoning and fine-tuning injection.

Deep neural networks (DNNs) are vulnerable to backdoor attacks, where adversaries can manipulate pre-trained backdoored DNNs and their corresponding applications to produce poisoned outputs when presented with poisoned inputs but behave normally with clean inputs. However, current backdoor attacks in DNNs exhibit certain limitations in terms of stealthiness and steadiness, such as pattern-fixed or even human-perceptible triggers, separated latent space features, and neurons with abnormal behavior. These limitations make them easily detectable or eliminable as backdoor defenses in DNNs advance. To bridge this gap, this paper introduces a novel backdoor attack in DNNs, named FeatureTrojan, which enables pre-trained backdoored DNNs to demonstrate enhanced stealthiness and steadiness. Specifically, unlike previous fixed-pattern triggers that are both human-perceptible and directly added to clean inputs, FeatureTrojan embeds dynamic triggers in latent space features of clean inputs and uses them to guide pre-trained diffusion generative models in generating corresponding poisoned inputs with human-imperceptible triggers. Then, in contrast to retraining pre-trained clean DNNs, FeatureTrojan fine-tunes them, effectively injecting backdoors and producing pre-trained backdoored DNNs while keeping the poisoned parameters close to their clean counterparts. Extensive experiments are conducted on multiple datasets and DNNs to demonstrate that FeatureTrojan can endow pre-trained backdoored DNNs with better stealthiness and steadiness. Compared with the current state-of-the-art backdoor attacks in DNNs, the total average Attack Success Rate (ASR) under various backdoor defenses in DNNs is absolutely improved by  ∼ 30.0%. The experimental code is publicly available at https://github.com/Afreadyang/FeatureTrojan.