It's going to be a month since Shellshock's vulnerability was public and it is also being said that Shellshock is worst than the Heartbleed vulnerability. After the public release of the Shellshock bug, many firms and organizations have patched the vulnerabilities but there are still tonnes of servers that are vulnerable to the Shellshock bug.
Shellshock bug was located in the Bash shell of the Linux operating system which resided for 20 years. It was deployed on the configuration which makes the bug severity critical and was also easy to exploit.
Hackers are targetting SMTP servers because the mail server is often left untouched (outdated or with little concerns) for a long time.
CSO said on the post that they have found one of the IRC servers used to host the bots, and they mentioned that it had 160 compromised servers connected to it on 24 October.