Towards understanding the security of modern image captchas and underground captcha-solving services

Haiqin Weng, Binbin Zhao, Shouling Ji, Jianhai Chen, Ting Wang, Qinming He, Raheem Beyah

Research output: Contribution to journalArticlepeer-review

25 Scopus citations


Image captchas have recently become very popular and are widely deployed across the Internet to defend against abusive programs. However, the ever-advancing capabilities of computer vision have gradually diminished the security of image captchas and made them vulnerable to attack. In this paper, we first classify the currently popular image captchas into three categories: selection-based captchas, slide-based captchas, and click-based captchas. Second, we propose simple yet powerful attack frameworks against each of these categories of image captchas. Third, we systematically evaluate our attack frameworks against 10 popular real-world image captchas, including captchas from,, and Fourth, we compare our attacks against nine online image recognition services and against human labors from eight underground captcha-solving services. Our evaluation results show that (1) each of the popular image captchas that we study is vulnerable to our attacks; (2) our attacks yield the highest captcha-breaking success rate compared with state-of-the-art methods in almost all scenarios; and (3) our attacks achieve almost as high a success rate as human labor while being much faster. Based on our evaluation, we identify some design flaws in these popular schemes, along with some best practices and design principles for more secure captchas. We also examine the underground market for captcha-solving services, identifying 152 such services. We then seek to measure this underground market with data from these services. Our findings shed light on understanding the scale, impact, and commercial landscape of the underground market for captcha solving.

Original languageEnglish (US)
Article number8665729
Pages (from-to)118-144
Number of pages27
JournalBig Data Mining and Analytics
Issue number2
StatePublished - Jun 2019

All Science Journal Classification (ASJC) codes

  • Artificial Intelligence
  • Information Systems
  • Computer Networks and Communications
  • Computer Science Applications


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