Securing Online Social Networks against bad Bots based on a Necklace CAPTCHA approach

Protecting Online Social Networks (OSNs) against the new generation of the automated software tools (i.e. Social Bots) is an increasingly important lack from the security and privacy perspectives. Social bots is automated software scripts, which able to carry out dangerous activities in OSNs, such as auto-sharing and posting, auto-sending friend requests, auto-harvesting private information,etc. Indeed, These malicious activities harm the users' reputation and privacy as well as they challenge the Social Network Provider (SNP) as a security issue. In this paper, we introduce a novel anti-bot mechanism called Necklace CAPTCHA for securing Online Social Networks against the smart generation of social bots. The Necklace CAPTCHA is an Image-based CAPTCHA which depends on the Necklace Graph approach to generate its tests. Our results proved that the Necklace CAPTCHA is an effective anti-bot mechanism from the usability and security perspectives, such that it achieved efficiency rate about 93%, solving time around 24 seconds, and low probability value of guessing attack success rate about 1.35% in average. Compared with other CAPTCHAs, the Necklace CAPTCHA is a strong competitive approach regarding the usability and security metrics.