PushDo/Cutwail Spam Botnet bruteforce

Details of BNVL-2019-0022

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Name : 
PushDo/Cutwail Spam Botnet bruteforce
Related Links: https://en.wikipedia.org/wiki/Cutwail_botnet



The attacking IP is infected with the PushDo/Cutwail Spam Botnet and tries to bruteforce SMTP.

That idiosyncratic and invalid EHLO argument “ylmf-pc” is a known fingerprint of a widespread spamming botnet known as “PushDo” (and sometimes alternatively “Cutwail”). As shown in your transcript, it is mostly trying password guessing to authenticate and send mail through your server. Keeping it from sending spam to or through your server is easy, because of three characteristics:

It actually sends the EHLO command before it receives the SMTP banner sent by the target MTA, a behavior that any modern MTA worth using (i.e. maybe not Exchange or QMail) can detect as a definitive indication of an unwelcome, pure-spam client. No legitimate SMTP client exhibits such “fast-talking” behavior, since it breaks basic SMTP functionality.

The string “ylmf-pc” is not a valid EHLO argument, because it is not a fully qualified hostname or bracket-enclosed IP address literal. No legitimate SMTP client will ever use that argument. As with the fast-talk behavior, any decent MTA can be configured to reject mail from clients using an inherently invalid EHLO argument.

pattern: “HELO”: “ylmf-pc”