A study in detecting network intruders
Uninvited Guests
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The nightmare of any admin is a user who can't resisting clicking on an unknown attachment labeled Application.exe. This article draws on a real-world example to show how you can use built-in Linux resources to detect unauthorized traffic that might have been invited in by a trigger-happy user.
If a network monitoring process detects malware, a system administrator needs to identify the affected systems and contain the damage. A customer from a Microsoft-heavy environment recently came to me with a problem. Many of the clients on his network were infected with malware. Because the malware was quite sophisticated, the virus scanner did not help detect it. He wanted my help with finding all the infected clients.
To make things even more exciting, several versions of the malware appeared on the network. The variations in form meant that it was not easy to detect the malware using simple pattern matching with a filesystem scan. Fortunately, although the attackers were good at infiltration, they were not very skilled at connecting back to their Command and Control (C&C) server. This article describes our investigation and offers some tips on how to respond to similar attacks.
The Malware
The first generation of the malware attempted to connect to Telnet servers in Asia via TCP port 23. Since the Telnet protocol is hardly ever used on today's networks, this attack was quickly noticed. A single tcpdump [1] command directed at the gateway returned some initial hits:
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