How Far Are We from Real Synonym Substitution Attacks?

In this paper, we explore the following question: how far are we from real synonym substitution attacks (SSAs). We approach this question by examining how SSAs replace words in the original sentence and show that there are still unresolved obstacles that make current SSAs generate invalid adversarial samples. We reveal that four widely used word substitution methods generate a large fraction of invalid substitution words that are ungrammatical or do not preserve the original sentence's semantics. Next, we show that the semantic and grammatical constraints used in SSAs for detecting invalid word replacements are highly insufficient in detecting invalid adversarial samples. Our work is an important stepping stone to constructing better SSAs in the future.