1 INTRODUCTION

2 BACKGROUND KNOWLEDGE

2.1 Voice Recognition Systems

2.2 Audio Features

2.3 Backdoor Attacks

2.4 Comparison between Backdoor Attacks and Other Adversarial Attacks on DNNs

3 EVALUATION CRITERIA

3.1 Effectiveness

3.2 Efficiency

3.3 Stealthiness

3.4 Practicality

4 BACKDOOR ATTACKS AGAINST VRSs

5 DISCUSSION ABOUT BACKDOOR DEFENSES AGAINST VRSs

6 OPEN ISSUES AND FUTURE RESEARCH DIRECTIONS

7 CONCLUSION