1 INTRODUCTION  
2 HISTORY AND PROBLEMS OF CAPTCHAS  
3 ADVERSARIAL EXAMPLES AND ADVERSARIAL CAPTCHAS  
4 TEXT-BASED AND IMAGE-BASED ADVERSARIAL CAPTCHAS  
5 AUDIO ADVERSARIAL CAPTCHAS  
6 CHALLENGES OF ADVERSARIAL CAPTCHAS  
7 POSSIBLE DIRECTION FOR FUTURE RESEARCH  
8 CONCLUSION