BadCLIP: Dual-Embedding Guided Backdoor Attack on Multimodal Contrastive Learning
Authors: Siyuan Liang, Mingli Zhu, Aishan Liu, Baoyuan Wu, Xiaochun Cao, Ee-Chien Chang

While existing backdoor attacks have successfully infected multimodal contrastive learning models such as CLIP they can be easily countered by specialized backdoor defenses for MCL models. This paper reveals the threats in this practical scenario and introduces the BadCLIP attack which is resistant to backdoor detection and model fine-tuning defenses. To achieve this we draw motivations from the perspective of the Bayesian rule and propose a dual-embedding guided framework for backdoor attacks. Specifically we ensure that visual trigger patterns approximate the textual target semantics in the embedding space making it challenging to detect the subtle parameter variations induced by backdoor learning on such natural trigger patterns. Additionally we optimize the visual trigger patterns to align the poisoned samples with target vision features in order to hinder backdoor unlearning through clean fine-tuning. Our experiments show a significant improvement in attack success rate (+45.3 % ASR) over current leading methods even against state-of-the-art backdoor defenses highlighting our attack's effectiveness in various scenarios including downstream tasks. Our codes can be found at https://github.com/LiangSiyuan21/BadCLIP.