  The field of adversarial textual attack has significantly grown over the last
few years, where the commonly considered objective is to craft adversarial
examples (AEs) that can successfully fool the target model. However, the
imperceptibility of attacks, which is also essential for practical attackers,
is often left out by previous studies. In consequence, the crafted AEs tend to
have obvious structural and semantic differences from the original
human-written text, making them easily perceptible. In this work, we advocate
leveraging multi-objectivization to address such issue. Specifically, we
reformulate the problem of crafting AEs as a multi-objective optimization
problem, where the attack imperceptibility is considered as an auxiliary
objective. Then, we propose a simple yet effective evolutionary algorithm,
dubbed HydraText, to solve this problem. To the best of our knowledge,
HydraText is currently the only approach that can be effectively applied to
both score-based and decision-based attack settings. Exhaustive experiments
involving 44237 instances demonstrate that HydraText consistently achieves
competitive attack success rates and better attack imperceptibility than the
recently proposed attack approaches. A human evaluation study also shows that
the AEs crafted by HydraText are more indistinguishable from human-written
text. Finally, these AEs exhibit good transferability and can bring notable
robustness improvement to the target model by adversarial training.
