I published the following diary on isc.sans.edu: “Who Are Those Bots?“:

I’m operating a mail server for multiple domains. This server is regularly targeted by bots that launch brute-force attacks to try to steal credentials. They try a list of common usernames but they also try targeted ones based on a list of email addresses that have been crawled. The mail server is protected by several security controls. One of them is an automatic blocking of offending IP addresses to slow down such kinds of attacks (brute-force) but I noticed that this technique was less and less relevant. Indeed, when a new wave of connections is launched, they are coming from a lot of different IP addresses that just test a few credentials and, therefore, do not trigger the automatic blocking… [Read more]

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