  Neural text ranking models have witnessed significant advancement and are
increasingly being deployed in practice. Unfortunately, they also inherit
adversarial vulnerabilities of general neural models, which have been detected
but remain underexplored by prior studies. Moreover, the inherit adversarial
vulnerabilities might be leveraged by blackhat SEO to defeat better-protected
search engines. In this study, we propose an imitation adversarial attack on
black-box neural passage ranking models. We first show that the target passage
ranking model can be transparentized and imitated by enumerating critical
queries/candidates and then train a ranking imitation model. Leveraging the
ranking imitation model, we can elaborately manipulate the ranking results and
transfer the manipulation attack to the target ranking model. For this purpose,
we propose an innovative gradient-based attack method, empowered by the
pairwise objective function, to generate adversarial triggers, which causes
premeditated disorderliness with very few tokens. To equip the trigger
camouflages, we add the next sentence prediction loss and the language model
fluency constraint to the objective function. Experimental results on passage
ranking demonstrate the effectiveness of the ranking imitation attack model and
adversarial triggers against various SOTA neural ranking models. Furthermore,
various mitigation analyses and human evaluation show the effectiveness of
camouflages when facing potential mitigation approaches. To motivate other
scholars to further investigate this novel and important problem, we make the
experiment data and code publicly available.
