1 INTRODUCTION  
2 HISTORY AND PROBLEMS OF CAPTCHAS  
2.1 Gimpy, EZ-Gimpy, and Gimpy-r  
2.2 CAPTCHAs in phpBB  
2.3 reCAPTCHA  
3 ADVERSARIAL EXAMPLES AND ADVERSARIAL CAPTCHAS  
3.1 Semantic adversarial CAPTCHAs  
3.2 Non-semantic adversarial CAPTCHAs  
4 TEXT-BASED AND IMAGE-BASED ADVERSARIAL CAPTCHAS  
4.1 Gradient-based methods  
4.2 Optimization-based methods  
4.3 Evolutionary algorithm methods  
4.4 Generation model methods  
4.5 Methods that successfully applied to text-based and image-based CAPTCHAs  
5 AUDIO ADVERSARIAL CAPTCHAS  
5.1 Parameter-tuning methods  
5.2 Optimization-based methods  
5.3 Gradient-based methods  
5.4 Hardware-targeted methods  
5.5 Genetic algorithm methods  
5.6 Methods that successfully applied to audio adversarial CAPTCHAs  
6 CHALLENGES OF ADVERSARIAL CAPTCHAS  
6.1 Adversarial Training  
6.2 Image Filtering  
6.3 Defensive Distillation  
6.4 Gradient Masking  
6.5 Incremental Learning  
7 POSSIBLE DIRECTION FOR FUTURE RESEARCH  
7.1 Constructing surrogate models  
7.2 Improving transfer ability  
7.3 Unexploited method to generate adversarial examples  
8 CONCLUSION