Zombies and botnets - detecting "crowd surges" in logs and network traffic
Tenable released a TASL script for the Log Correlation Engine that can use netflow, sniffed network sessions, firewall logs and even network IDS logs to help identify botnets, maleware and zombie networks.
The basic premiss is that for certain protocols like SSH, Telnet, IRC and custom high-port control mechanisms, if we have a "large" user population suddenly all decide to visit an IP address on the other side of the world, this could indicate a "phone home" or some sort of control mechanism.
At the Tenable blog.
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