Mirai spreads to devices by scanning the internet for unprotected systems, and then recruiting them to carry out hacks. Mirai can turn unsecured devices such as web cameras, routers, phones and other hackable internet-connected devices into 'bots'. The identity of the hacker behind the malware source code is unknown. It’s malware code has also been published on an online hacker forum.

http://www.dailymail.co.uk/sciencetech/article-3819882/Are-risk-Experts-fear-spike-hacking-code-used-one-largest-cyber-attacks-history-released.html